That is an internal forge issue with the fluid sorting. The only way to fix it is NBTedit/NBTExplorer.
First make a backup of you world safe!
The use NBTExplorer or NBTEdit to open your safegames level.dat.
There at Forge/DefaultFluidList search for oxygen and delete the entry. Deleting the full list might also work, but in that case the allready stored fluids in your world might change into different ones.