It is pretty interesting because it is one of the first overdrive pedals which sounds more like an amplifier and less like an effect pedal.
Of course, the opinions vary, but in my honest opinion the Fulltone OCD reacts very interactive on the way how the guitar played.
The OCD is based on the circuit of the Voodoo Lab Overdrive or rather the DOD Overdrive preamp.
Instead of the usual clipping diodes which can be found in overdrive pedals, the OCD uses Mosfet transistors which make it quite dynamically.
In difference to the clipping diodes, that are switched against ground, the Mosfets are switched to the input of the OP amp.
This is the reason why the OCD reacts enormously touch sensitive.
Fulltone was one of the first boutique manufacturers who built clones of several effect pedals. And you can also find numerous clones of their OCD.
One of the cheapest is the Joyo Ultimate Drive which in fact offers a pretty good quality and gets really close to the sound of the original.
Actually, the differences between the circuits of the different versions of the OCD (numbered from 1.1 to 1.7) which have been invented through the years, are even marginal.
And- as so often- the first versions are the most famous and most sought-after ones.
At the moment, the version 2 of the pedal (for the first time with additional buffer) is offered in a slightly altered enclosure.
The OCD is able to cover nuances from a slight overdrive up to distortion while the sound of a slightly overdriven amplifier is preserved all the time.
A small switch allows to chose between two basic sounds: the HP (sounds more British, a bit like a Marshall amp) and the LP (which sounds more neutral with less prominent mids).