When I was using OD (nowdays am using my amp, Budda SD, for OD) I also paired up the two. For me the difference is as you describe but also the Zen is...I have to be careful here, it's not "middy" so much as "the exact right kind of mids to cut beautifully but not be boxy", and OCD is not at all middy to me. The OCD can give more of a (being careful again, I don't mean it sounds just like but in the direction of..) SLO-100 kind of OD.OCD, I find the Zen a little dark on its own and the OCD is a little bright but together they are almost perfect.
That's funny. I feel the opposite way about these pedals, the OCD feels "darker" to me. I do feel like the Zen has more midrange than the OCD. I have a v1 OCD.OCD, I find the Zen a little dark on its own and the OCD is a little bright but together they are almost perfect.