What happens if you add too much O2 at pitch time? As an example, consider a situation where there's a 23L batch of 1.045 wort @ 15C. The brewer pitches the yeast, then uses a diffusion stone with pure O2 and leaves it running for 3 times as long as normal (say 5 minutes instead of 90 seconds).
What Coalminer said, or more to the point MHB's post of O'Rouke's paper, or really the test results of Moll.
By the way, the Yeast book has similar results, but the 'Observed' O2 ppm in 20 L 18.7 Plato wort at 24C indicates that 120 seconds O2 at 1L/min using a 0.5 micron sintered stone gave 14.08ppm O2. At 60 seconds they got 9.20 ppm and at 30 seconds they got 5.12 ppm. These experiments can be found on pg.77 of that book and were conducted by White Labs. The thing with those experiments though is that it doesn't state how long those O2 concentrations lasted or at what point the DO tests were conducted (ie immediately or after X time period). Pg. 81 of the same book shows results of DO tests on different commercial breweries in which the maximum DO level found was 35.8 ppm O2, but that was a brewery that ran O2 at 25-30 mins into 12.8 Plato wort at 6L/min (10bbl volume, but unstated temperature or O2 injection method and again it is unknown at what stage this DO test was taken). In those same tests, most of the other breweries ran O2 at similar or more L/min rates with much less O2 concentration as a result (lowest 5 ppm after 40 min O2 at 6L/min in 12.5 Plato wort!). It makes me wonder about both the consistency of the DO test methods and the O2 injection methods of the different breweries. Either way the dissolved O2, at larger time frames than you propose (20-80 mins), of those tested gave an average (mean) result of 10.6 ppm O2, though the median average is 6.54 ppm O2. Now some of that O2 may have been metabolised by the yeast during the 20-80 mins of injection, but again the tests and O2 injection methods are not explained well enough to be able to bring them into the Home Brew environment. They go into how few of these breweries measured DO and were guessing/estimating the O2 levels based on flow rate and time (shows how wrong they were). They also state that once the O2 was brought up to the recommended 8-10 ppm levels they got fuller and quicker attenuation with healthy generations of yeast for re-pitching (there's a take away message in this, though I wouldn't be getting a DO meter for the cost. Watch out for WEAL's future posting on the matter as he has recently ordered one, so we may get a glimpse of his results next year).
I would surmise from the various literature (including O'Rouke's in the above linked post) that, if you ran the O2 for 5 minutes instead of 90 seconds, you'd
probably get a very small amount more O2 into solution (probably, as I would predict it would mostly dissipate back to levels as posted by MHB fairly quickly [unless there was head pressure]). IF the O2 remained in higher concentration long enough for the yeast to metabolise it [ie you had head pressure to keep it there for 20-40mins to allow the yeast time to fully metabolise it], then you would get a healthier happier yeast and a quicker better attenuated beer with a resultant decrease in Ester production, but an increase in Fusel alcohol production (Laere et al., 2008). I seriously doubt that you would cause oxidation at 5 mins, though if there are more oxidising agents (ie Lipids) and/or yeast are sluggish and/or are way under pitched it could happen. Maybe, but we are talking theoretical questions here right?