You've done a wonderful job with world building and mechanics, it definitely is an extremely engaging narrative. I definitely agree that the oxygen drain could have a bit more of a sense of urgency to it. Also there was a passage where the player character gains a health boost, but honestly having both health and oxygen may be a bit redundant as the mechanics are now. I think the oxygen drain idea was a really great one and it could stand really strong on it's own with a few tweaks. In the future if you were going to include both adding more narrative elements that utilize both of them could help with that but that may lead to more work with having to balance them. Overall this is a great game and it was a super interesting story to play through! :)