like the title says How Do you Refill Oxygen tanks?

Beats me, I would like to know too. 

You don't, they're useless in the current version of the mod.

The space suits automatically fill up when you go back to the overworld.

thxs

well in old versions you could auto-refil oxygen tank s with the refill station, but only SWDBae's could obtain them

 you still can (well i can)