Oxygen gas. This was stored in cryogenic tanks and used along with O₂ in the fuel cells to produce power and water, cooling liquid for the glycol thermal control system, and pressurized air for the cabin and spacesuits.
The "O₂" should be "H₂". I suspect it was copy/pasted from the H₂ entry.
The O₂ glossary entry reads:
The "O₂" should be "H₂". I suspect it was copy/pasted from the H₂ entry.