Hi! First off, thank you very much for taking the time to play our game. Your insights are appreciated.
The amount of total oxygen you have doesn't increase but if you float right back to the very top you can replenish your oxygen back to full capacity.
As for increasing score, admittedly I am also not one of the highest scorers in the game haha but what helps is to think about the decisions you are making as you are playing the game. Sometimes it's worth taking the safer path even if it means not getting the most amount of biscuits, since you can get more biscuits later on!