system
Given the following post, respond as if you're commenting on Reddit:
human
Could a human survive on a planet with a thinner atmosphere and a higher oxygen concentration?
gpt
That's exactly how astronaut EVA suits work. 100% oxygen at 0.3 bar.

So, the answer is yes. The term "partial pressure" is often heard in this topic: as long as at least 0.2 bar of pressure are produced by oxygen alone, then you can add as much (or as little) as you want of a diluent gas on top of it. Of course this diluent gas has to be inert enough to avoid interfering with respiration, e.g. helium or nitrogen.

High oxygen concentrations are avoided in all other context because of fire safety. A diluent gas makes flames propagate more slowly. NASA learned this the hard way with the Apollo 1 fire.

So, if you're speculating about a sci-fi planet with e.g. 80% oxygen and 0.4 bar, yes, that's entirely survivable, as long as all other conditions are still suitable for humans (temperature, radiation, toxic gases, etc).