I very fun and well made game.
The controls are difficult because 'z' is right between wasd and space.
Cool idea and good use of the theme. Kind of monotonous but enjoyable. Great art. It was a bit confusing at the beginning figuring out I needed to stand near trees to get oxygen. Picking up items annoyingly only works if you are standing in just the right spot. The 2 in O2 should be subscript not superscript.
