I just spend about 3 hours comparing a V.4 OCD to my Xotic BB Bass preamp. I can't decide which I like more.
The Xotic is more transparent, and retains ALL the low end. Nothing is lost, but the Fulltone OCD kind of sounds a little more open and is more versatile. I love the TONE knob, it helps control those annoying highs that can come from using oD pedals. The OCD does cut some bass, but I think in the mix it may cut through better.
Both are great pedals BTW.
I'd be happy with any one of them.
I have only tried the OCD V.4, so I can't comment on other versions, but you can read this:
http://www.pedalarea.com/ocd.htm