I've had OCD since I was a kid (diagnosed + re-diagnosed), just got diagnosed BDD a few weeks ago. Imo, I think since both are obsessive and involve a type of rumination/compulsive "checking", it only makes sense that they can go hand-in-hand. Lots of OCD has to do with perfectionism, and especially living in a patriarchal state, I can understand why it's also super common to develop BDD as well. Comorbidity is up to 43%.

I think the key thing to know they're 'linked' is to pay attention to the compulsions and insight. For me I feel the same anxiety for both types of checking (checking the mirror, googling, or even physical rituals for ocd). If it's something the person can't seem to stop, it's obsessive, a habit, and needs treatment. They're separate things, but have so much in common. BDD has poor insight - they think that they look ugly/not right, no matter what people say. OCD can be both - many of us know we're being irrational, but still believe our fears, so our insight is there but impaired. But we try. When you obsess, you keep checking, The difference is slim, and it's complicated... but in my experience, one is just more physical than the other. This inspired me to learn more about them both lol