You can't breathe that oxygen, however, unless you blend it with 4 parts of nitrogen or something similarly more massive than the oxygen to slow down fires.
Low pressure (100-200 mbar) pure oxygen behaves similarly to low partial pressure atmosphere in inert diluate (i.e. air). It's not perfect -- things still burn faster without the diluent -- but it's not as dangerous as pure O2 at sea level pressure, nor is it like the high-pressure pure O2 environment in the Apollo 1 test.
Also, N2 (and especially Ar) aren't consumed to an appreciable degree. They can be recycled in an artificial life support system (as they are, and have been, for decades).
https://www.discovermagazine.com/the-sciences/why-apollo-had...
It would be a comic turn of fate if we succeeded to turn the Moon into an oxygen-rich atmosphere, only to discover that we can’t land on it because every single rocket we sent on it burns upon landing.
That is something like ~7 the Oxygen density of normal air. Things burn hot and fast in that environment.