There is a pretty significant bug in the game: when you have used up all of your shots, if the last zombie you hit is a Mortar, then as soon as the Mortar hits the ground, the level ends ... even if that Mortar hit another zombie and killed it. (I've seen a level end just as another Mortar was firing.) So, whenever you can, avoid using your last shot on a Mortar - if the Mortar can't kill the remaining zombies at once, the level will end.

You'll see right away that the 100 levels in OMGZ are arranged in a triangle so that you work your way from left to right; also, levels are generally more difficult at the bottom than at the top. You'll notice this as soon as you've completed a couple of levels and unlocked more: you'll most likely end up unlocking levels toward the top, until you reach a point going to the right that you can't clear a level and thus can't unlock any more.

NOTE: Once you've completed all 100 levels at least once, you can actually play a level that you haven't yet unlocked, as long as you've beaten Level 1 first, even on a new save. If you get a medal on that level, you can unlock other levels ... but you won't be able to see your progress on a "locked" level until you unlock it. If you haven't beaten all levels yet, you can't play locked levels.

When you select a level to play, you'll see the basic count of each type of zombie in that level; you can use this to determine which upgrades to buy if you have available cash. Keep in mind that Population upgrades will increase the number of the appropriate zombie type, but won't be reflected in these counts: for example, #100 will always show 1 of every time, even if you have Population maxed for all zombies. Also, Population will never add a zombie type to a level; it will only increase the count of zombies already there. (Note: Barrels are not shown on this screen.)

The medal you get for a level determines what you unlock in the column to the right of that level:

• Bronze (60-74%): the next level up and to the right
  
• Silver (75-89%): the next level to the right
  
• Gold (90-99%): the next level down and to the right

Medals are cumulative, so a Gold will unlock all three adjacent levels (if they aren't already unlocked). Getting a Bronze on #1 will unlock #2; a Silver will unlock #2 and #3; a Gold (or Platinum) will unlock #2, #3 and #4.

You can see how the bottom half of the triangle is difficult to unlock at first: without a Gold, you can't unlock levels down and to the right, and without a Silver, you can only unlock levels up and to the right.

Each medal earns you cash to spend on upgrades: like medals, money is cumulative, but money is not awarded twice, so if you have a Bronze on one level and you get a Platinum later, you get $14,000, not $15,000 (because you already got $1000 for the Bronze).

• Bronze: $1000
  
• Silver: $2000 - getting a Silver on a new level is worth $3000
  
• Gold: $3500 - getting a Gold on a new level is worth $6500
  
• Platinum (100%): $5000 - getting a Platinum on a new level is worth $11,500

Upgrades cost $2000 per upgrade per type, and increase by $2000 for each upgrade you buy for a specific type: if you've bought four upgrades for Soldiers, the next Soldier upgrade will cost you $10,000. Each type can be upgraded 10 times, for a total of 100 upgrades per playthrough and a total cost of $1,100,000. That seems like a lot, but if you get a Platinum medal on all 100 levels, you'll have enough to buy all upgrades with $50,000 left over.

In-game, the skull (RB on your controller) toggles color for zombies; you can play with this off, I guess, but it's a lot more work. (There isn't a color-blindness option.) The plus (LB) toggles health bars; again, playing with this off makes the game a lot more work.

Each zombie has between one and five ticks of health. When a zombie's health reaches 0, it's destroyed, and its "death thing" is triggered. Zombies can take damage from Rifle shots, fire, and other zombies.
The zombies in the first picture have 1 to 3 ticks of health: the Zapper and Cop near the middle have 1, the Bloated at the bottom has 2, and the other zombies have 3. Killing either of the Bloated zombies in the middle of the screen would kill the Cop (by doing one tick of damage) and maybe the Zapper (if the Bloated's range was upgraded enough; that might be close enough for even a basic Bloated to kill), but nobody else. Shooting the Cop zombie would probably not kill the Bloated with three ticks (because he would be too close to the Cop when the Cop fires) but would probably kill the Bloated with two (a basic Cop does at least two damage, and the Bloated would be in line with the Cop's shot).
The zombies in the second picture have everything except 2 ticks: Bloated zombies with 1 and 3 ticks, Civilian and Cop zombies with 4 ticks back-to-back, and Civilian and Bloated zombies with 5 ticks (full bars). This would be a difficult situation if your Rifle can only do 4 ticks of damage: you could try to shoot the Civilian with 4 or the Cop, but if you missed and hit the Bloated with a full bar, you wouldn't kill anyone. A better move is to shoot the Bloated with 3 ticks, which should a) do one tick of damage to that small group, b) killing the one-tick Bloated, which will then do another tick of damage to all those zombies, leaving you with two two-tick zombies and one three-tick zombie.

There are eight types of zombies:
Standard zombies that explode when killed, damaging all zombies within a given radius. Upgrades increase the number of Civilians on the screen, the radius of the blast, and the damage it does. Initially, they do one tick of damage; maxed out, they will kill anything in their range.
Strength: Large groups. Civilians are always good for starting chain reactions, particularly as you increase their damage.
Weakness: Range. Even with maxed-out range, they won't damage zombies more than a zombie-length or so away.

Larger zombies that explode when killed, but with greater range than Civilians. Unlike all other zombies, Bloated zombies never do more than one tick of damage, so upgrades just affect population and range.
Strength: Chains of Bloated scattered among other zombies. Because of their greater range, if you get it set up right, you can kill one Bloated and trigger a chain reaction that wipes out most of the group, assuming the other zombies are all near the Bloated zombies.
Weakness: Damage. Because they can never do more than one tick, Bloated on their own are mostly useless.

Zombies that fire one bullet when killed, damaging zombies along the line of fire until the maximum number of zombies have been hit, regardless of whether or not each zombie along the way is killed. Upgrades increase population, penetration (more zombies can be hit), and damage (Cops do 2 ticks initially). Note that the bullet is fired in the direction that the Cop is facing when it dies; this may not be the direction it was facing when it was first damaged. Also note that it will not damage a zombie directly in front of the Cop; this is presumably because the Cop is holding its gun out from its body. Cop zombies are right-handed, so the bullet will come from the right side of their body. This is important when the Cop is moving from top to bottom; you want the target zombie(s) to be slightly to the Cop's right, as thin zombies directly above or below may escape the bullet.
Strength: Long range. Fully-upgraded Cops can clear out large swaths of zombies, even on other parts of the level, and obstacles that prevent zombies from moving do not block bullets.
Weakness: Damage is done only in one direction and starting at a short distance away. If the Cop is facing away from other zombies, or if the only zombie is at point-blank range, its bullet is wasted.

Like Cops, but they fire multiple rounds in random directions within a cone of vision in front of them, and those bullets ricochet off zombies and Barrels until the maximum number of ricochets occurs, doing two ticks of damage to every target hit every time it's hit. Upgrades increase population, number of shots, and number of ricochets per shot.
Strength: Crowds, particularly crowds of Soldiers. They can clear out a level easily when they are upgraded sufficiently; you'll need this to get a couple of the achievements.
Weakness: Like the Cop, damage is done only in the direction the Soldier is facing, but worse, only along random lines. Asking a Soldier to hit a specific zombie is asking for a lucky roll of the dice.

When killed, instead of exploding, Bilious zombies melt, creating a pool that damages all zombies that enter it (but not barrels that it envelops). There are some levels where the pool will actually seep up and over a wall into an adjacent area! Upgrades increase population, duration of the pool, and the damage per second it does.
Strength: Crowds, particularly when fully upgraded. One well-timed shot on a Bilious can bring down a huge group of zombies, especially when other Bilious zombies are part of the group and the pool expands. The pool can actually do quite a bit of damage even when not fully upgraded because of the duration.
Weakness: The pool does not move, and the hitbox for it is not very tall, so a zombie can move just a few steps above or below and miss it completely. Also, on early levels, the pool disappears quickly, so zombies aren't usually affected by a Bilious death unless they're right next to it.

When killed, they launch a shell (a brain?) into the air that lands on a random part of the level, damaging all zombies around the landing spot. Upgrades increase population, blast radius, and damage (which starts at 2).
Strength: Levels that are mostly full. An upgraded brain can do a lot of damage, and if other Mortar Zombies are in the blast radius, that can set off a chain reaction that can take out zombies pretty much anywhere on the screen (though not all at once).
Weakness: Total randomness. The brain really can fall anywhere on the screen. When you're down to only a few zombies, Mortars are pretty much useless.

When killed, they sprint a short distance in the direction they're facing (long enough to scream twice, one long and one short). When their sprint is over, or if they run into an obstacle, they explode. Upgrades increase population, blast radius, and damage (which starts at 2): sprint distance remains unchanged. Note that Runners will run through a Bilious pool without taking damage ... after all, they're already dead again.
Strength: Levels with connected sections. Runners are perfect for carrying a chain reaction from one section to another. Note that the blast, like most damage, will not pass through walls.
Weakness: Short-range damage. Unless a Runner is facing a nearby wall or other obstacle, you can't make them explode in place.

Like Cops, when killed, Zappers shoot one time (with a taser instead of a normal gun), but unlike Cops, they shoot in a random direction. If their shot hits, a line from the Zapper to the target stays electrified for a certain amount of time, damaging both the target (unless it's a Barrel; Barrels do not take damage from Zappers) and anything crossing the path of the line. The target (if it's a zombie) turns randomly while being zapped, so it may die facing a new direction. Damage to the target occurs all at once after the duration expires; damage to zombies crossing the line occurs while they are touching the line. Upgrades increase duration of the line, damage, and population.
Strength: Crowds (increasing the number of zombies walking into the electrified line) and distance. An ideal shot hits a zombie at maximum range, allowing many zombies to cross the line in the meantime.
Weakness: Total randomness plus zero penetration. Even a shot in a crowd of zombies may not be much good if there is a zombie immediately adjacent and that one gets hit.

Barrels can be found on many levels. They always have one tick of health; when damaged (by anything other than a Zapper or Bilious), they explode, setting fire to all zombies in the blast radius. Zombies take damage until the duration of the fire expires. Upgrades increase population, range, and damage.
Unlike with zombies, you are not told whether or not there are Barrels on a level.
Strength: Large crowds. A Barrel's blast radius is fairly large, especially when maxed out, and it can set fire to zombies that are hard to reach otherwise: on the other side of walls, up a cliff, etc.
Weakness: Time of effect. Even with damage maxed out, zombies with full health when hit will not die immediately and will continue to move as they burn. Often, that means zombies with directional attacks end up facing away from where you wanted them to be when they died.

Unless you have a near-photographic memory, you won't know which levels have barrels and which don't until you try one, because there's no other way within the game to know this, other than by consulting the Barrel Table:


has Barrels
does not have Barrels

Upgrades always are unlocked in the same order for each type, so it helps to know what you're going to get when you buy one, particularly if you are stuck on a level and need help with it. This is most important for the Rifle, less so for zombies and Barrels.

A couple of quick tips:

• Generally speaking, avoid buying upgrades for zombies that aren't on the level you're about to attempt. You can't undo an upgrade, and you don't want to be in a situation where you need a huge amount of luck to proceed.
  
• The Rifle has the only upgrades that are used on every level, so that should be your priority ... not that you need all 10 upgrades before getting anything else - among other reasons, some levels you won't be able to clear even with 6 shots until you upgrade zombies - but it usually works better to buy a Rifle upgrade than a zombie or Barrel upgrade at the same price.

Here's the key for the next two tables - I had to break the table into sections because it wouldn't fit in one:

• Because you start off with only 3 shots, the most important upgrade is the first Rifle upgrade; it will give you an extra 33% (a fourth shot), and that'll help you get through the early levels, especially when you don't have much in the way of other upgrades.
  
• Always be aware of the power of your Rifle, particularly early in a playthrough. When you only have 3 or 4 shots at minimum power, as soon as you try levels with zombies who have more than one tick of health, you can blow the entire level simply by wasting one shot. Note that you can estimate your Rifle damage based on the number of shots you start a level with:
  
  Shots

  Damage

  3

  1

  4

  1-2

  5

  2-3

  6

  3-5
• Remember that Cop bullets can only penetrate 5 zombies at full range. To maximize their damage, instead of shooting a Cop, look for another zombie in the Cop's line of fire that would kill the Cop, and shoot it first.
  
• On levels with both Civilians and Barrels, keep in mind how you want to start your chain reaction: Civilians provide an instant explosion, while Barrels stretch the effects farther if the zombies caught in the blast have enough health to burn for a while. Usually, you'll be better off shooting the Civilian to trigger the Barrel, because that will kill directional zombies the way they're facing as you shoot the Civilian, and the Barrel may reach zombies out of the Civilian's area of effect.
  
• Don't waste bullets! Remember that Mortar zombies launch a brain that disappears for a second or two; wait for it to come down before you shoot. Also, the electrified line that a Zapper shoots can be hard to see, and you don't want to shoot a zombie the Zapper already hit. (Exception: if there's a cluster of zombies that's unusually large for the level or that represents a big percentage of remaining zombies. It may be better to "waste" a bullet and ensure you kill that group than to end up with an extra bullet and too many zombies to kill.)
  
• Watch the ratio of zombies to bullets, and don't forget to account for the power of your Rifle. Obviously when you have at least one bullet for every remaining zombie, you can get your 100% easily, but if you get down to 2-to-1 or sometimes even 3-to-1, you can still get your Platinum medal if you are patient and wait for the right setup.
  
• Conversely, don't feel like you have to wait 10 minutes for the perfect alignment to clear a level. There's no penalty for replaying a level, and you'll always keep the best progress you've made, plus any bonus $$ for improving your medal. If the three remaining zombies aren't wandering close enough to each other to take them all out with your last bullet, just shoot one and try the level again.
  
• When zombies overlap, it can be difficult to hit the zombie you want; it's best to avoid this situation, especially if you're low on bullets. If you have to shoot - say, the zombies have grouped and you don't have many bullets left - then aim at a zombie moving away from the overlap, so that even if the zombie keeps moving, you're more likely to hit it than the others.
  
• If you're trying to hit a Barrel, remember that you can shoot the flame and still blow up the Barrel.

Just by completing the game once, you'll get the following achievements:

Sweeter Than Gold - for getting a Platinum medal on any level
You Made It... Right? - for clearing a level in the far right column and seeing an ending
Happy Shopper - for getting all Upgrades in one category
Shopaholic - for getting all Upgrades in all categories
No Stone Unturned... - for getting Platinum medals on all levels
Flawless Victory - for seeing the best ending; which level in the final column has it? (You'll know it when you see it, and not just by the achievement popping.)

Note that Shopaholic will pop before you complete the game - as noted in the comments, you can afford all upgrades if you have 90 platinum and 10 gold. If you feel Excel-ish, you can also hit it with 95 platinum and 3 silver, 95 platinum and 2 silver and 2 bronze, or 96 platinum. There's probably a couple of other combinations in there to keep you occupied on your next 19 playthroughs ... just keep in mind that you get $50K extra every time through. As soon as you hit $1.1M, you've got enough for all upgrades.

You can then get Back In The Line by going to the Extras menu and selecting Prestige, then clicking Yes to start your second playthrough.

You'll probably get these on the first time through anyway, but just in case ...

Frying Tonight - for killing 5 other zombies with a Zapper. To get this achievement, fully upgrade the Zapper's Duration (which requires the $20,000 upgrade), then shoot a Zapper on a crowded level.

Barrel of Laughs - for completing a level by only shooting Barrels. You can pair this with Feel The Rush and complete a level with a single bullet by shooting a Barrel. In the next section, you'll see some levels where this is easy to do.

Line 'Em Up - for killing five zombies with a Cop. To get this achievement, fully upgrade the Cop's Penetration (again, the $20,000 upgrade), then shoot a Cop on a crowded level.

Bang!! And The Zombies Are Gone!! - for prestiging the game 20 times. Every level, 100%, 20 times. Get started ... there's no easy way to do this one.

#1: The Beginning
Your goal here should be at least a Silver medal, so that you earn enough to buy the first Rifle upgrade, which will give you 4 shots instead of 3 from this point forward. It may take a few attempts to get one where zombies cluster together well enough for three chain reactions to wipe out 75% of them, but it's worth it ... and even better, if you get a Gold, you can buy three upgrades before you try #2!
#4: Alive Inside
Easy to get a Bronze, surprisingly hard to get a Platinum. The problem is that you only get Bilious and Zappers, and neither can trigger a Barrel, so you have to blow those up yourself. If a Barrel isn't surrounded by zombies, look for a cluster of Bilious and shoot one: the rest will die quickly because they all have just one tick of health. If you can't get a cluster or a well-placed Barrel, you may need to retry.
#15: Bagged and Tagged
Probably the first level that presents a real challenge. It's difficult because it's split into three sections, with only Cop bullets able to cross the barriers. When your Ammo is maxed, two shots per section gives you a chance, but early on, with 3-4 bullets, you'll struggle. Once you have upgraded Cop bullet penetration, go back to this level and watch for Cops near one end of a section and pointing toward the other end. Their bullets should trigger Civilians along the way, cleaning up the larger groups and hopefully leaving you with just a few zombies you can pick off to get 100%.

#22: No Way Through
The first level where no zombie starts with only one tick of health. Don't bother with this level until you upgrade your Rifle's Damage to 2 or higher.

#32: Presumed Undead
The first level where no zombie starts with fewer than three ticks of health (all have three on this level). Even with Barrels present, you won't be able to kill enough zombies to make this worthwhile. Until both your Rifle and Civilians can do three ticks of damage, it will be difficult to get a medal on this level.
#33: Missing Persons
Like #32, all zombies have three ticks of health, but there are Cops in addition to Barrels, so if you've upgraded Cop's Damage, they will kill anything they shoot, and you might scratch out a Bronze or a Silver even if you haven't yet upgraded your Rifle's Damage to 3.

Outside the Redfield A&E Hospital.
#41: DNR
Like #32, all zombies have three ticks of health. With no Civilians and no Cops or Soldiers, this is worth skipping until your Rifle is upgraded.

Levels on an open cargo lift; while they seem easy, a smaller number of zombies means that after initial explosions, the survivors are spread out and harder to kill.
#46: Elevation
Because of the large number of Zappers, when you shoot a Barrel and kill a number of them at once, you'll hear a sound remarkably like the sound your PC makes when something really, really bad just happened. That's because of the number of Zappers shooting out tasers simultaneously - it's not because OMGZ just killed your PC.

Levels where it only takes one or two shots for the bullets to be flying.
#50: Angel of Death
This is a great level to use for Feel the Rush and Barrel of Laughs because of the number of Soldiers. Look for a Barrel with a number of zombies around it, shoot it, and wait. Remember to wait for all of the Soldier shots to be fired; Feel the Rush requires that you only shoot the Rifle once.
#52: Divine Tragedy
Similar to #50, with Barrels and Soldiers, but Bilious instead of Bloated zombies. Even if you don't one-shot these levels, a Platinum should be easy to get, and that will help you get upgrades to work on the bottom half of the triangle.

Levels where a long, thick wall running left to right splits the area in half. The top half is shaped almost like Chevrolet's logo; the bottom is mostly rectangular, with a small extension in the middle of the bottom side of the rectangle.
#59: Doors Opening
With no Barrels, it's harder to kill groups of Zombies, so focus more on shooting Cops and Soldiers pointing toward other zombies. As you upgrade Civilian's Population, this level will become easier to clear.
#60: End of the Line
Tricky if you don't yet have your Rifle upgraded to do 4 Damage, as most zombies have 4 ticks of health. (There are usually a couple of Soldiers with less health, plus Barrels to weaken the others.) If the Cops are distributed so that they are mostly facing other zombies, you'll be fine. If they are facing toward the outside walls, you'll end up with zombies scattered across the level and no way to clean them up. This level might be one where it's better to shoot early and make sure you destroy a key group of zombies than to wait and see if a chain reaction takes them out.
#61: Undying Load
Basically the same as #60.

Levels that are divided into four rooms, one in each corner, although there are wide passages from left to right and narrow ones from top to bottom.
#62: Tested On Animals
The key here is to send Runners down the passages to spread a chain reaction and have Soldiers shoot across the level to keep it going. You may even get lucky and have a Zapper or two hit a long-range target and fry a few extra zombies!
#64: No Pets
The problem with this level is that you have no Civilians and very few Cops and Soldiers, plus Zappers who will spin the Cops and Soldiers around and make them useless. To clear this, you need to use Cops and Soldiers as best you can, even if it means not waiting for a chain reaction to finish. Even so, you may still need a group of Bloated that can destroy each other (one with one tick of health, one or more with two, etc.) to clear out what the Cops and Soldiers and Barrels don't.

Levels with cells separated by walls, with no more than two cells connected.
#77: Jailbreak
This level is difficult because the cells keep zombies divided into small groups and because there are no Civilians. Your best bet here is to find a Soldier near the center of the level and use him to start a chain reaction.
#78: Cell Damage
Like Jailbreak, but not as hard because of the presence of Barrels and Civilians. Again, look for a Soldier near the center, or a Civilian near a Soldier with the Soldier pointing toward the center; this is better than a Barrel because you want the Soldier to shoot toward the center, where he's more likely to hit other zombies, and with a Barrel, the Soldier could turn away while he's burning and end up shooting off-screen.

#100: The Truth is In There
All the zombies here have max health, so unless your Rifle does max damage, you should probably hold off. Other things that make this possibly the hardest level to clear (as it should be, being #100) are that there aren't many zombies around, even with everyone's Population maxed, and that there aren't many long stretches for Cops to shoot down, so every Cop is valuable - you can't afford to have one shoot off the screen.