I know some people do it, but I've never added O2 after pitching. Most of my beers are in the 1.08+ range. IMO, aeration at the outset and a big healthy pitch of yeast will get you what you need.
But, if you choose to go with a second does of O2 I would use a very low flow rate. I try to set it so I can just see the bubbles coming out of the stone. If the surface of the wort is roiling, your flow rate is too high.