Okay, I must be honest. I think you've done a bit of a disservice to your son. As a divorced woman, you live with him as a partner at this point. He feels dependent on you. OCD is treatable and many, MANY people have OCD and function just fine in life. He's living under your roof as a person somewhere between being a man/child or being your surrogate partner. And your daughter has always been the responsible, hard working one who acts like an adult. Ya, she's resentful of it. I kind of get where she is coming from.
I have a friend whose brother also lived like this. My friend's mother died. Her brother then was completely lost. He'd never lived as a true adult. This is what is going on with your son. It's hard to see that when we feel comfortable with the situation.
For his best interest, you need to help him separate from you. This will be very hard for you as you are probably on some level as dependent on this as he is.
Your daughter is right. You've parented your son to the point of keeping him in this place while she's had to be a functioning adult and she's tired of that discrepancy. She's really giving you a wake up call. She didn't handle it very well, but that's what this is.
Good luck
Sorry that your daughter "told you" to seek counseling, since it's irritating to be told to do something instead of having it just be a suggestion, since really, counseling is the thing to do in this circumstance, especially with the Christmas holiday imminent. You've got a lot to look at the relationship with your daughter and the relationship with your son, possibly even your ex. Your son is pretty old to be taken care of like a teenager, gotta say. And your daughter feels like all she got for not demanding your attention and making a mess of her life was less attention and less resources. It also sounds like the difficulties in her life are making his apparently easy ride on your money really irritating. Not that you're Bill Gates, but it can seem very unfair to the not-screwing-up child that the screwing-up child gets all the time and attention and that he's had more of your financial resources can seem like the last straw. Find yourself a counselor and don't wait. The calendar says, move on this or the holidays will be miserable. A counselor or therapist will help a lot.