https://en.wikipedia.org/wiki/Hyperoxia
This was a screenshot I took months ago while watching a Geology Hub upload on YT. It was a lightbulb moment for my understanding of mass extinction events, (the largest was 250ma). I’ve referenced this multiple times, so thought I might share. Perhaps you find it as interesting as I do.
You can absolutely breathe higher partial oxygen pressure unaided for a long time. Hyperoxia isn’t all that lethal and definitely not quickly, if you’re only visiting, there’s no problem.
And if you want to live there, you should be much more worried about all the brand new diseases you don’t have immunity to, or the bugs that are bigger than you.
The low oxygen is definitely a problem, especially if you need to run away from the stuff mentioned above.