So, I was looking to refill my supply of bottle caps and ran across "Oxygen Absorbing" bottle caps. The implication is that their absorbency will reduce oxidation in the bottled beer. If you're already "capping on foam", etc. do these make any actual difference or is this just another gimmick?
Yes; I'd say they do.
Even when capping on foam, you're still going to end up with some O2 in the bottles. This usually isn't enough to turn a beer bad, but depending on how long you're planning to age the beer it can make a difference. I'd always use oxygen-absorbing caps on higher gravity beers like barleywines and imperial stouts that I plan to keep in the bottle for 6+ months.
Two things to keep in mind when using them:
At my LHBS, the oxygen absorbing caps don't cost much more than regular caps so I always buy them. YMMV, so take the style of beer you're bottling into consideration and decide from there if there's a huge price difference.