The 808 circuit that Maxon uses is essentially the "authentic" or "original" Tubescreamer circuit, except that they generally use better components than most of the units on the market. It's the same circuit that several manufacturers on the boutique level offer. What most folks aren't aware of is that it's an extremely similar circuit to the Boss SD-1 OD, and that the Boss unit can be modded to an exact duplicate of the 808 with just a few small changes and (optionally) some upgrades on certain components in terms of quality. If you know how to do it, you can save a whole heckuva lot of money doing it that way. Or, like I do these days, you could build 'em from scratch yourself and get exactly the sound you want.
The OCD, like all overdrives and distortion pedals, has diodes that produce clipping of the signal. However, it is not a "distortion pedal", per se, in that while it does produce an unusually large amount of clipping for an overdrive, it does not produce the compression levels that distortion pedals nearly always do. That's why the OCD has such an open and (mostly) transparent sound. And unlike almost all distortion pedals, the OCD allows you to dial that distortion way down to clean up the signal. Most distortion pedals don't let you remove the distortion below a certain minimal amount and they don't let you remove any of the compression.
The OCD is just a really powerful overdrive. While it will produce gain levels that reach distortion pedal territory, it's far more versatile than almost any true distortion pedal. Like has been mentioned, you do have to be careful when using it with a Rectifier because the OCD also boosts the low-mid (@ 200Hz) frequencies responsible for mud, which the Recto is already famous for overproducing. I've found from personal experience that it's nothing an EQ can't completely tame.
Both the 808 and the OCD are good overdrive circuits. They each have strengths and weaknesses that the other doesn't cover. For me, the ultimate answer was, "Why choose?" I have a version 4 OCD that I really like and I have half a dozen variations on the 808 circuit including an exact duplicate model.