In the article on Apollo command module design, I read:

  • The cabin atmosphere at launch was changed to 60% oxygen and 40% nitrogen at sea-level pressure: 14.7 psi (10.1 N/cm2). During ascent the cabin rapidly vented down to 5 psi (3.4 N/cm2), releasing approximately 2/3 of the gas originally present at launch. The vent then closed and the environmental control system maintained a nominal cabin pressure of 5 psi (3.4 N/cm2) as the spacecraft continued into vacuum. The cabin was then very slowly purged (vented to space and simultaneously replaced with 100% oxygen), so the nitrogen concentration fell asymptotically to zero over the next day. Although the new cabin launch atmosphere was significantly safer than 100% oxygen, it still contained almost three times the amount of oxygen present in ordinary sea level air (20.9% oxygen). This was necessary to ensure a sufficient partial pressure of oxygen when the astronauts removed their helmets after reaching orbit. (60% of 5 psi is 3 psi, compared to 20.9% of 14.7 psi (10.1 N/cm2), or 3.07 psi (2.12 N/cm2) in sea-level air.)

  • The environment within the astronauts' pressure suits was not changed. Because of the rapid drop in cabin (and suit) pressures during ascent, decompression sickness was likely unless the nitrogen had been purged from the astronauts' tissues prior to launch. They would still breathe pure oxygen, starting several hours before launch, until they removed their helmets on orbit. Avoiding the "bends" was considered worth the residual risk of an oxygen-accelerated fire within a suit.

(emphasis mine)

The article on Oxygen Toxicity has a following table: Oxygen poisoning at 90 feet (27 m) in the dry in 36 subjects in order of performance

"Several hours" seem like much longer than the 96 minutes the most persistent test subject withstood, of exposure to pure oxygen at atmospheric pressure, and risk of oxygen-accelerated fire within a suit seems like the least of worries the astronauts would have.

How were they dealing with oxygen toxicity before and during the launch, before the pressure was vented to 5 psi?

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    $\begingroup$ Isn't there a factor of 5 between the table (oxygen at 3 atmospheres pressure, i.e. 15 times more oxygen than in normal air) and the environment in the command module (max. 0.6 atmospheres oxygen partial pressure)? $\endgroup$ – asdfex Jan 11 '16 at 19:39
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    $\begingroup$ @asdfex: Oh, I had assumed 27m altitude (near sea level pressure), not depth... Still, the factor would be closer to factor of 3 for the few hours before launch - the command module is only vented in space. (other than that it would be more, the command module was launched at 60% oxygen content and refilled to 100% over the first day in space) $\endgroup$ – SF. Jan 11 '16 at 21:57
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    $\begingroup$ Your chart is for a substantial depth in the ocean. At sea level the safe exposure period is a lot longer (I want to say 16 hours but I won't swear to it) than they would need before the pressure drops to safe levels. $\endgroup$ – Loren Pechtel Jan 12 '16 at 0:41
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    $\begingroup$ @SF. Looks like my memory was wrong. Safety limit is 5 hours: decodoppler.files.wordpress.com/2009/02/noaa-tables.jpg $\endgroup$ – Loren Pechtel Jan 12 '16 at 20:21
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    $\begingroup$ @SF The limits for 1bar pure oxygen is given in : en.wikipedia.org/wiki/Oxygen_toxicity#Signs_and_symptoms "One or two days of exposure without oxygen breaks are needed to cause such damage". So 5 to 16 hours as safety limit are possible depending on the safety margin. $\endgroup$ – Uwe Sep 30 '17 at 21:20

Reading the listed Wikipedia article, it is evident that these experiments were done at pressure, the same pressure that would be experienced at 90 feet of water. I've been trying to find a better source, and I looked at the source article for that chart. It contains the following:

... were safe breathing oxygen ... for at least three hours at 3 atmospheres

It seems the toxicity only really happens once one gets above 3 atmospheres, the safe exposure for 4 atm is around 30 minutes.

While it isn't recommended for everyone, there seems to be no adverse effects to breathing pure oxygen at atmospheric pressures for several hours, as was done for the Apollo program.

  • $\begingroup$ 90' of (fresh) water is roughly 4 Atm, assuming there's 1 atm above the water. The symptoms listed for 6-10 minutes aren't life-threatening and probably aren't even completely debilitating, but I wouldn't really call them "safe" for anybody who needs to be doing anything, especially in what could easily become and emergency situation. $\endgroup$ – CBHacking Jan 17 '16 at 11:37
  • $\begingroup$ In diving the limit for breathing pure oxygen is 1.6 bar or a depth of 6 m water. At 1.7 bar or more there may be convulsions. "at least three hours at 3 atmospheres" is considered unsafe in the water and should be used in recompression chambers only with supervision personel breathing no pure oxygen but compressed air. $\endgroup$ – Uwe Jul 11 '18 at 20:38
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    $\begingroup$ It seems the article I used was from 1947, so it might be a bit out of date... $\endgroup$ – PearsonArtPhoto Jul 11 '18 at 21:19

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