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No problem! I do actually have an idea that just popped into my head.

If you know the game Random Access Mayhem, one of the playable characters uses a flamethrower, and if you use it too long you start to overheat and eventually go kaboom.

Perhaps something like that could work? Holding down the flamethrower for too long causing heat to build up in your suit, making you rapidly burn oxygen till you cool down again? (Probably no need to include the kaboom part tho :p)

Gives an extra avenue for upgrades, based on both managing and maybe even using the heat buildup. But that's just my own personal idea.

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.