Goodness me. Why does everything have to be so 'offensive' these days? People can't seem to open their mouths without causing a domino effect of wide eyed pearl clutching. When did we all become so precious and delicate? 
I mean, I understand to a degree on the one hand. OCD is an awful mental illness and if you do suffer from it (in whatever capacity), I can imagine how overhearing people trivialising it day-to-day might make your eye twitch a little. But it's just a thing people say. There was no spite or malice behind what your colleague said, OP. There is a vast difference between your coworker making a comment about himself, and nastily making such a comment directly to a person he knows suffers from OCD. I personally cannot understand getting so worked up over this, and I say that as a person who has suffered with on and off bouts of seriously debilitating, poor mental health for 15+ years.
My 9 year old daughter's been diagnosed with Dyslexia. Dyslexia, not unlike OCD, affects people in a myriad of different ways and can range from mild to severe. My DD's left me brokenhearted at times because she often feels stupid and knows she's bottom of the class for things like spelling and maths. She's a smart wee cookie, but she has her struggles. I don't get worked up every time I overhear someone seemingly self-diagnose themselves as 'a bit Dyslexic' whenever they make a spelling mistake, for instance. In my mind, if it comforts somebody to label themselves as such to ease the embarrassment, distress or anxiety of said struggle, then more power to 'em. If it gives somebody a way of coping with whatever it is that's caused a flare of upset/unease, I fail to understand why it's an issue for anybody else.
It's all very well for a group of strangers on the internet to question this man's word choice, but if he's feeling so uneasy about a messy desk that he has to vocalise a label to explain it, then that to me suggests his unease could be very real and shouldn't be mocked, belittled or branded offensive by others. People are just doing their best to get through the day. My advice? Give the poor bloke a break and save your gasps for something worthy of one.
It's a going-against-the-grain YABU from me, I'm afraid 