GAMEPLAY: The controls were overexplained--I can grasp "WASD - Move". The concept of oxygen being a limited resource in a maze is both a reasonable expansion of the genre and an interesting dynamic. The zombies didn't seem to add much, though. This is entirely personal, but I'd prefer a proper maze without enemies.
VISUALS: Being consistent with pixel size isn't vital, but having a 5x difference like with the controls menu (not to mention the anti-aliased font) throws things off a bit. So does the lamp not being pixel art at all, seemingly.
AUDIO: The constant zombie groaning, while on theme, got tired quickly. I appreciate having a hit sound for when sucessfully attacking.
THEME: Oxygen being a resource and being able to pick up and attack with almost anything certainly fit the bill.