This is fairly easy and straightforward.
You must be in a Co-Op game for this to work.
Early in the game you will be given a Oz Kit from Janey Springs, this will allow you to hold O
2 in your inventory. If a teammate is down and is Fight for you Life you can revive them using O
2.
Simply hold
instead of
to revive using O
2 instead.
Note: this will use up your own oxygen supply, so its better to do this indoors or within an oxygen bubble. Or if you have Fragtrap as your character it doesn't matter because he doesn't drain oxygen while being out in a vacuum.
If you don't have anyone to play with or don't feel like playing with randoms in a public match, simply sign in a second controller into a guest account and make the other person stand in lava. Every time they go down you revive them.