My school: 1 OMM required rotation.
Rotation quality depends. We have some cranial sites where you sit and transmit good vibes for a month. Others are at FM clinics where there's near zero OMM focus.
I did my FM rotation alongside a classmate on his OMM rotation. We both did bread and butter FM with a young attending who couldn't help but chuckle when he told us to do OMM which was reserved for obviously MSK complaints. And we were only allowed to do ST, ME, or CS. He had a decent sized clinic, so it wasn't a big deal that we took an extra room for 10 extra minutes.
I was told to do CS/ME/ST on one pt with lots of anxiety due to serious life issues (eg divorce, daughter had abusive partner, etc). We sat and chatted for like 10 minutes while I worked on her neck for tension HAs and shoulder pain. She said both were severely reduced after the session. Was it the talking or the OMM? I don't know. But the OMM gives a great excuse for sitting with a patient for a little bit of extra time without making it seem like an interrogation.
I've since gone ~3 more rotations without thinking about OMM.