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.

Terrestrial Dinosaurs/Aerial Dinosaurs

Terrestrial/Aerial dinosaurs usually have low oxygen because they live on land, away from water. 10 is often the default oxygen for most dinosaurs.Except for the Pteranodon, which has more than 10 oxygen.

Aquatic Dinosaurs

The majority of Aquatic dinosaurs have infinite oxygen. Their natural element is water, so they instead have a moistness gauge for when they're out of the water. A few exceptions to this rule are Mosasaurus and Pliosaurus. They have both oxygen and moistness, potentially as a balance for their powerful combat statistics. Their oxygen is much greater than 10, however. Mosasaurus' oxygen is 600, meaning it only needs to go the surface every so often for air.

Hybrid Dinosaurs

Hybrid refers to dinosaurs that are hybrids(as their name suggests). These dinosaurs include the Albino Terror and Megavore. While the Albino Terror has only 10 oxygen and can't swim well, the Megavore can swim as well as creatures like Spinosaurus. The Megavore, however, only has a mere 30 oxygen which makes it unable to stay in the water for too long.