Act with patience knowing that it’s not the person’s fault that s/he has OCD and might have even possibly been born with it and is handling a test directly from Hashem-rather the test is from the perspective of having a sickness or the test of patience and being organized and being mevater etc…. -That’s between the OCD patient and Hashem directly who gave him the nisayon.
Don’t try to prove their obsession (s) wrong
Don’t say ‘calm down’
Don’t say ‘what, are you worried about crossing the street too?”
If you’re able to be a listening ear, go for it. If you’re not, then politely say that you wish you knew what to say and how to help, but that you don’t know, but that you respect them and don’t think any less of them because of their condition
Exposure Response Prevention works wonders but it is hard. Try it slowly. Give the person the space and time to deal with OCD. Remember that OCD is generally chronic so the goal should be about managing it not completely getting rid of it necessarily. Obviously, a good therapist is crucial. Not all therapists treat OCD properly. Try to find one that does.
Speak to a therapist and find the best way to get them into therapy. OCD can seriously ruin a persons life and they may be in denial that they have a problem. Therapy is absolutely necessary.