If you don't oxygenate on brew day, is it too late or can you oxygenate late? If so, are there limits?
The yeast need the oxygen to grow and reproduce, which is important for the first stage of primary fermentation when the yeast is multiplying and inhabiting your wort, which you want to happen as quickly as possible to avoid risk of infection when the wort is cool, exposed to air and does not yet have a protective yeast head. Unless you're brewing a high gravity beer, you should avoid introduction of oxygen at any point after this as it will lead to staling.
If you forget to oxygenate on brew day, I bet that chances are there is enough oxygen present for some yeast growth, simply by the physical act of transfering wort from boiler to fermentation vessel, but this depends on your technique. If you started your yeast off in a starter, even better.
See this website for more technical info: http://www.howtobrew.com/section1/chapter6-9-3.html
Once fermentation has started it is usually not recommended to add oxygen. Exceptions: When brewing a high alcohol beer you may add oxygen up to 12 hours after pitching, but not afterwords.
Yeast will consume oxygen during the initial fermentation phase. After that the oxygen stays around to stale your beer.
My experience as as professional brewer and home brewer leads me to suggest that oxygen is not detrimental to beer flavor unless yeast activity is in decline (i.e., after vigorous fermentation has subsided). So if you have forgotten to oxygenate, go ahead and do so as long as your beer has yet to achieve high krausen.