I run an in-line oxygenation system. I place it after the chiller so that the oxygen solubility and uptake is higher. There is no way that you could ever get any sort of back flow of wort into the oxygen system when the regulator is cracked open because the pump can only supply a few feet of head. Remember that 1 psi is about 2.3 ft of water column (head). After you've pumped through the chiller, you've probably only got 5 or 6 feet of head loss to get into the fermenter (assuming the fermenter is at the same level as the pump). That means the oxygen system only needs to provide 2 or 3 psi to overcome the head in the tubing.
A check valve would be nice. But considering that system would quickly reach pneumatic-lock (like hydraulic-lock) if the wort backflowed into the oxygen line, that wort isn't going to go far. Its just not a reality for my system.
I have about 20 ft of tubing after the airstone, so the oxygen has a decent amount of time to dissolve in the wort. I only trickle the oxygen into the wort...enough so that I can actually see that there is a bit of foam streaming in the tubing during the wort transfer. I probably get 15 to 20 5-gal batches with the typical little red bottle. It would be nice if the little red bottles were food-grade, but there probably isn't much else in that oxygen. I suppose there could be some other gases, but not much else.