if dinosaurs are similar in breathing to humans

Current living dinosaurs are much more efficient at extracting oxygen from air than practically anything else in the planet.

Birds've got a unidirectional respiratory system that ensures oxygenated air is constantly flowing through their lungs (unlike, for instance, us mammals, who must empty our lungs of spent air before we can fill them again), and a system of air sacs to keep the air constantly flowing.

While fossil records of the earliest dinosaurs show no evidence of air sacs, later ones do, suggesting that bird-like respiratory systems evolved multiple times in parallel in different branches.

Sauropods in particular might have had even more complex air sac systems than modern birds, which could explain how they managed to grow so large (i.e., they were full of air, and might have been even more efficient when it comes to breathing, though their long necks might have offset the balance in the opposite direction).

Dinosaurs would have been perfectly fine with current oxygen levels.