[eru]

Oxygen becomes toxic to humans at higher partial pressures. (That is already a concern to divers.)

I do not have the numbers in my head to tell whether dinosaur-era levels of oxygen would be enough to cause problems. And in any case, the oxygen level would take probably at least a few hundred years to rise.

Fun fact: The rate of oxygen production by plants is big enough to re-oxygenize the atmosphere to current levels in around 2000 years, should all oxygen be wiped off the planet. (Assuming everything else stays the same, including CO2 is kept at current levels.)