Ok, I'll try to be more nuanced than some other contributors, however I do understand where they're coming from and I generally agree with the spirit.
We all agree : this is not normal AT ALL. Unlike some other posters on here, I personally have had the occasional conversation with both male and female friends about porn in general. A notable example is a casual (male) friend who brought up, during a discussion about feminism, his own conflicted relationship with certain kinds of porn. At other points in my life there were members of the porn industry in our circle of friends, so it did come up at times. Porn is part of a lot of people's lives, and it is therefore strictly normal that it will come up in conversations. I find it especially important for people to address the concerns they might have about it if they have any and need to talk about it, too. But as with any subject relating to intimacy and sexuality it shouldn't be put up ostensibly and be the sole object of conversation, especially if the speaker is a man, addressing a woman and his particular obsession with a given type of porn or porn star.
You did mention that this individual has severe OCD, and that therefore explains the, well, obsessive behavior to an extent.
Yes, his OCD could be centered around this particular individual... but in my experience OCD is more about a given set of behaviours that one repeats in order to control a given aspect of their life.
But in this case the object of the obsession is a real person, who exists and has feelings, and he is objectifying this person in a pretty creepy way that actually kind of echoes that of a stalker. And through that, given the relationship to porn, he is kind of dehumanizing women as a whole... and that's kind of scary.
As others have said, you have a right to set your boundaries and refuse to engage in conversations that make you uncomfortable or object to being in a space where a woman like you is being turned into a thing.
If you really want to take this upon yourself to an extent, you should tell that person that they should definitely engage in counselling and by all means get the crap off those porn subs as they're a dangerous echo chambers. But if it was me I wouldn't go further and I would definitely avoid this person as this seems pretty scary.