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.
Well, you can breathe it, although it will result in oxygen toxicity. This doesn't have anything to do with fires, and the gas doesn't have to be "more massive" than O2 to retard fires either.
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).
Apollo used pure oxygen, and skylab was 3/4 oxygen, 1/4 nitrogen. For context, this was done to simplify systems, and lower the pressure in both the spacesuits and spacecraft; low-pressure in the spacesuit is important because it makes the suit more flexible.
Didn’t the full-oxygen Apollo 1 burn in a horrific accident while still on launchpad, and is the reason why doors can now open from the outside? One scratch and even iron burns in a 100% oxygen atmosphere. Honor to those 3 astronauts.
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.
The problem was that it was full pressure, 1 atmosphere of pure oxygen. That was stupid and was only really done on that test, because it was a sea level mock-up. The actual spacecraft operated at just the normal oxygen partial pressure, but with no other trace gasses aside from exhaled CO2.
Apollo 1 was particularly bad because they were attempting to simulate relative space pressure on the ground. So they pushed the internal pressure up to 20 psi of pure O2 (sea level is ~15 psi).
That is something like ~7 the Oxygen density of normal air. Things burn hot and fast in that environment.
