Glad you liked the atmosphere! Presentation was certainly a big focus for me on this project.
As far as the balancing of resources, that could have used some more work as you discovered lol. At the moment I think there are three oxygen bottles that can be found around the main rooms you initially have access to before finding the wrench.
The oxygen mechanic was mainly there as I had intended for a much longer underwater segment near the end of the game. But of course I ran out of time.