The book Yeast will tell you most everything you ever wanted to know about the subject. At the start, yeast eats up all available oxygen to reproduce. More yeast means a better and faster starting fermentation. The book recommends about 45 seconds of oxygen injection for the basic 5 gallon homebrew batch. I have been using our host's oxygenation kit for some time with definitely noticeable results.
Although pure O2 injection is best method, according to the book anyway, any method to get air or oxygen in at the start of fermentation is a good thing.
It's important to note however, that after that time, oxygen is a bad thing. Your fermented beer will oxidize and not taste as good.
@dmtaylo2 Can you tell us more about not needing to aerate dry yeast. That's news to me too