I've heard two different things: the first being right before pitching the yeast (complete joy of homebrewing) and the second source says right after pitching (this website). So which is it? I'm inclined to believe right before pitching because you don't want to mess with the yeast once it starts doing its thing. Anyway any help is appreciated. Thanks.
It makes absolutely no difference between those 2 choices.
I don't know how much difference it makes, but pure oxygen may not be good for yeast. You want to aerate the wort as opposed to aerating the yeast.
I use O2 and pitch afterwards, but I've also aerated a second time several hours later for very strong beers.