The Oxygen Mask is one part of the Oxygen System used to breathe on planets without oxygen, the others being (Oxygen Gear and Oxygen Tanks).
- When you die with the gamerule
KeepInv = true, you still wear it but it no longer appears there. Leaving the game and restarting it can fix the visibility. (NOTE: this bug no longer occurs in the newest versions, at least from version 188.8.131.528 on).
- On the mask's icon there's no iron frame or support, but in-game there's an iron piece on the lower back of the player.
- Wearing an oxygen mask, gear, and tank does not prevent you from drowning in water. However, micdoodle8 has hinted to a hydra-attachment. See the forum thread.