For Apollo 1...

When crew died on the ground in an oxygen fire (and my sympathies to their family and friends). As a result, the USA changed its spacesuit and cabin pressure policy to a "shirtsleeves" environment, with huge decompression problems as a result.

There are no free lunches in space.

5 psi oxygen isn't a bad solution, with just a trace of inerts.

But you have to work towards it early on in a flight, not pretend it doesn't exist...

You could start on the ground at 1 bar and standard atmosphere and then transit to 0.3 bar and 95% oxygen at the same time as the stages are unfolding beneath you. It isn't pretty, or fun. Of course, you could alternatively have 0.3 bar 95% O2 on the stand, but then nobody could ever hope to open a hatch if something went worng. ever...

You need both Balls and Brains to be in the space business...