I use B-Bright, which I hear is very similar to OxyClean. It works very well. I plan on getting some OxyClean as soon as my B-Brite is done (very soon) as it is much cheaper.
I used to clean my kegs out as soon as they were kicked, but now I just leave them pressurized and diry and clean them the next time they need to be used. Never have had an issue getting the gunk out with B-Brite.