Here is more clear instructions Slightly modified from the original sources to apply to GOTY Enhanced rather than original Borderlands file structure.

1. From:

http://www.nexusmods.com/borderlands/mods/2/?tab=2&navtag=http%3A%2F%2Fwww.nexusmods.com%2Fborderlands%2Fajax%2Fmodfiles%2F%3Fid%3D2&pUp=1

download and unzip the Oasis Map Hub. This is the empty mod that enables the Oasis hub. None of the files in it overwrite anything in Borderlands. So, if you want to uninstall it, just reverse this process.

2. From its Binaries folder, copy the UE3ShaderCompileWorker.exe file to your Borderlands Binaries Win64 folder (steamapps\common\BorderlandsGOTYEnhanced\Binaries\Win64).

3. Copy the WillowGame folder to your Borderlands folder.(steamapps\common\BorderlandsGOTYEnhanced).

5. From:

https://code.google.com/archive/p/s6fulldlcfasttravels/downloads

download and unzip the FullDLCFastTravelHUB.rar file and unpack it.

6. Copy its mapslot6.umap file to your Borderlands Mapslots folder (...BorderlandsGOTYEnhanced\WillowGame\CookedPC\Maps\Mapslots). You'll be overwriting the existing mapslot6.umap file. But, that's just a placeholder from the Oasis Hub, above.

7. Just to be safe, make a copy of your SaveData folder (mine's at Documents\My Games\Borderlands Game of the Year\Binaries

Next we swap out a Borderlands level with Oasis. I used Arid_Arena_Coliseum_P for this like stated in the reddit post, but switching out any self contained level should work just fine.

-Navigate to BorderlandsGOTYEnhanced\WillowGame\CookedPC\Maps\Arid

-Rename or backup Arid_Arena_Coliseum_P.umap (Should be near top of folder)

-Copy W_Scrap_Oasis_P.umap from BorderlandsGOTYEnhanced\WillowGame\CookedPC\Maps\Oasis, place it into the Arid directory, and rename it to Arid_Arena_Coliseum_P.umap

-Run Borderlands, select a character as normal and enter the game. Travel to the Fyrestone Coliseum and the Oasis will load. Once loaded, a message should appear at the top of the screen stating that the Oasis outpost has been discovered.

-Now when you fast travel to Fyrestone Coliseum, it actually loads the Oasis. You Enter Number 6 and it has fast travel nodes to the DLC. Some are down the eleveator.

Unfortunatly you lose access to the actual Coliseum or whatever zone you decide to swap with the Oasis, but I never use the Coliseum anyway.

SOURCES:
https://www.reddit.com/r/Borderlands/comments/bao3iu/oasis_map_mod_with_borderlands_goty_enhanced

https://steamproxy.net/app/8980/discussions/0/133258593404433678/

Originally posted by https://steamproxy.net/id/Artophwar on the Steam forums. Made available here for easy search. All credit for the mod goes to the original creator Shawn "AmranX" Davison