Very fun and very promising!
The whole "health is ammo" mechanic with the oxygen meter is really cool, though once you pump enough upgrades into it, the mechanic also becomes largely inconsequential...
That being said, this is only a demo, so I assume you've already probably got ideas to make the gameplay loop more complex, like the spike-flesh mentioned in a previous comment.
I would maybe like to see some kind of "minimal oxygen" difficulty? Something to push the resource management on runs to its theoretical limit, if that turns out interesting.