You can link shelvs or whatever storage to the oxygen generator so it will pump oxygen into the storage and the items, such as oxygen tanks, will refill.
Oxygenite tanks can't be refilled since they have liquid oxygenite, not oxygen inside

Nah, It's just for storage

was it ever possible to fill up an oxygenite tank with oxygen?

I don't get it, Did they change how oxygenite works?