Oxygen is an in-game mechanic related to Moistness and having to do with water. Oxygen is an indicator of how long a dinosaur can survive underwater before they begin to take damage. Oxygen does not change when growing.
Terrestrial and Aerial
Terrestrial and Aerial dinosaurs usually have low oxygen because they primarily live and are adapted for land. 10 is often the default oxygen for most dinosaurs. Except for the Pteranodon, which has more than 10 oxygen, being semi-aquatic.
The majority of Aquatic dinosaurs have between 300 and 600 oxygen. This is mostly to balance their strength, although as they're sea reptiles they do need to go to the surface to breathe. Their natural element is water, so they instead have a moistness gauge for when they’re out of the water. Megalodon holds the record of them all, having 150 oxygen.
These dinosaurs that are Terrestrial or Aerial but can swim as well. This includes creatures like the Spinosaurus, Megavore, Sarcosuchus, Pteranodon, and other creatures. They usually have 120 oxygen, with the exception of Megavore and Pteranodon, that have 30 and 40, respectively.