The Fulltone OCD has been on my board for around ten years now and is my primary low gain overdrive pedal ever since I got it.
The pedal has a straightforward layout with volume, drive and tone knobs and a mini toggle switch for two different eq settings. Like most drive pedals, the volume knob controls the output level of the pedal - nothing new here. The drive knob controls the gain and will take you from almost clean to lush full vintage style high gain sound. The tone knob controls the tone obviously, but different pedals tone knobs tend to do things a bit differently. In the OCD, it sounds to me that the tone knob primarily boosts the high end as you turn it clockwise. The rest of the frequencies remain pretty much the same. The mini toggle switch allows you to select between HP (high peak?) and LP (low peak?) modes. HP provides more pronounced and aggressive high mids and sounds a bit gainier and LP sounds and feels spongier, warmer and more natural. Both modes sound fantastic and it's really a matter of which instrument you plug into it and more importantly, what is the character of the amp on the other side.
Although the pedal has good gain range, I use it primarily for low gain sounds, where to me it sounds most natural and dynamic. I would occasionally use the OCD after a fuzz and into a clean amp, to tame the fuzz a little and make it smoother and more in control. Another good use I found for it is a treble booster, where I set the gain all the way down, the tone control somewhere over 12 O'clock usually, and the volume as high as I need it to drive my preamp stage to the sweet spot. I would normally use the LP mode when using the pedal as a treble booster.
Pros
Sounds great with any guitar
Can be used as a treble booster
Like all Fulltone pedals, very well built
True bypass
Works great after a buffer
Works great after a booster
Cons
I wish it was more sensitive to playing dynamics and guitar volume
Doesn't stack very well with other overdrive pedals or overdriven amps
The higher gain settings are less useful to my taste. It sounds less natural, very spongy and it loses definition.
Verdict
The OCD is a great overdrive pedal to have on your board! It sounds and feels natural, has a useful range of tones and it pairs nicely with other pedals. Having said that, the OCD is definitely not for you if you are looking for a modern sounding aggressive or ultra high-gain overdrive! It has relatively moderate gain, it is saggy and doesn't have a tight low end at all.
The OCD has been around for many many years and has several different versions. Ever since it was born many similar full range overdrive pedals have been released and are definitely worth checking out if you are on the market for an overdrive pedal of that style. A few good ones I would consider:
Catalinbread Dirty Little Secret - tighter, more gain, more aggressive
Mesa Boogie Flux Drive - tighter, more modern, fuller sounding
Bogner Ecstasy Blue - not really the same kind of overdrive, but worth checking out for lower gain overdrives. More versatile, more modern, more gain range, extremely dynamic
Sound Sample
In the following track you can hear a Les Paul played through the OCD and into a Marshall JCM 2000 DSL 401.