NASA originally spec'd a mixed gas atmosphere for Apollo, it just got too heavy. After the fire in Apollo 1, they switched to mixed-gas at launch, with this gra

NASA originally spec'd a mixed gas atmosphere for Apollo, it just got too heavy. After the fire in Apollo 1, they switched to mixed-gas at launch, with this gradually bleeding out to pure O2 as the mission progressed. This was discussed in my comment in my linked submission.