I liked it. Simple concept but well executed.
I think it was interesting how there were two variables to keep track off for the player. But the fact oxigen was continuously going down was a bit frustrating. Health made sense design wise : it meant being efficient with your movements. But Oxigen was basically just a countdown.
A suggestion could be to give the player a way to hold their breath (another input?), which stop the oxygen from going down, but if held too long, you'll lose health faster. that would make the Oxygen more interactive.