The OCD was the first pedal to use Mosfets as clipping devices. Another first was their configuration as "hard clippers connected to v-ref" instead of to ground, as was the norm. These things contribute greatly as to why the OCD has such touch-sensitivity...why they react so well when the strings are hit hard or soft, and why they sound & feel different than all other pedals out there, 90% of which are just slight tweaks of the Tube Screamer.
Demo by That Pedal Show (EN)
Demo by intheblues (EN)
The Story of Fulltone OCD (EN)