The O2 Mask was acquired at Jowai Resort on Planet Pokitaru in Ratchet and Clank. The Jowai Resort owner promised to give Ratchet the mask if he and Clank escorted him to the other end of the resort and shot down the Blarg's dump ships with the Resort's Blarg ship. The mask allowed Ratchet to swim underwater for an unlimited amount of time without having to surface for air. It also allowed the Lombax to survive in poisonous atmospheres, such as on planet Orxon.
O2 Masks were very similar to Megacorp Helmets in Ratchet & Clank: Going Commando and Up Your Arsenal.
It was used by King Claude so he could fight for DreadZone in Ratchet: Deadlocked.
This gadget also appeared in Ratchet & Clank Future: Tools of Destruction and Ratchet & Clank Future: Quest for Booty along with the Grind Boots, Gravity Boots, and Swingshot from the beginning of the game without having to be found.
The O2 Mask appears in A Crack in Time, when Ratchet wanders moons in space. Some moons have highly-gaseous atmospheres therefore Ratchet needs an O2 Mask to continue.