Great game, music works great to build the atmosphere, and the oxygen concept is very interesting as a pseudo time limiter.
My only complaint about the game would balancing the fine line of giving the player not enough time or care to read the dialog, versus too much time to the point where the oxygen drain is so slow that there is no more danger.
The way I would go about combating this is keeping the slow drain so you can read the text, but having many more hazards that could drain health (if you click by without reading) or drain oxygen (trap rooms, etc.)
But still overall a good game with a fun mechanic that can be explore more, and sorry I couldn't help you with the background image, still scratching my head over that.