There was more oxygen in the air when the dinosaurs roamed, but not when insects (and other critters) first came on land.

https://forces.si.edu/atmosphere/02_02_06.html says

> As plants became firmly established on land, life once again had a major effect on Earth’s atmosphere during the Carboniferous Period. Oxygen made up 20 percent of the atmosphere—about today’s level—around 350 million years ago, and it rose to as much as 35 percent over the next 50 million years.