I always use O2, no matter what - dry, liquid, or repitch.
I just barely open the regulator until it starts to flow and then dial it back so it's just flowing. I let it go for about a minute or so, not very precisely timed. I like to end up with an good amount of bubbles all over the surface of the beer.
I don't know if I'd buy the O2 again, but now that I have the setup, it's a lot easier than pouring the wort back and forth a few times - no lifting!