This answer explains that from the time they suited up "in the Suit Lab before launch" until the time the capsule started depressurizing during ascent, the Apollo astronauts were breathing 100% oxygen at slightly above 1 bar (1 atmosphere).
100% Oxygen at 1 bar (100 kPa) partial pressure is toxic, or at least bad for you if you breathe it long enough. Presumably you'll be okay if it's not too long, so there must have been some time limit.
- How long were the Apollo astronauts allowed to continuously breathe 100% oxygen at 1 bar?
- Where there ever situations where the time was exceeded, and they literally had to "take a breather" and breathe something with a substantially lower oxygen partial pressure?