Quite addictive game, I must say. I really like the retro-style in graphics as well as in audio. One thing I especially want to point out is the fact, that your graphics all have the same pixel size. That is a thing that most people do not take care of but it makes a huge difference in credibility. Well done on that!
I think, adding some more dangers might give your game a little extra juice. Thinking about something like snakes or moles that move one block when the player is moving one block. Then, you must carefully think where to move to not hit the enemy.
Also, getting only one oxygen per upgrade feels a little slow. Maybe increase the number the more oxygen you already have?
All in all: Good job, liked it a lot