Some feedback sound or effect would be nice when you collect an oxygen tank.
Also since there were two weapons in the game, I wished I could have tried the pistol in the arena as well. Shotgun felt like it had maybe little bit too much spread or not enough projectiles. Anyways felt quite hard to hit targets with that.
Theme and the oxygen mechanic of the game reminded me a bit of the movie In Time. Also cool effects tied to the oxygens. I also liked the tutorial giving you a little story about the world. Ambitious project for a game jam. Good job!