I don't know of any who do exclusively OR. There are a number who started out that way. For example First Assist, OR Support (now Baron Healthcare), and Advanced Surgical (sister company of another agency). I could probably come up with a few more names if I thought longer. But they all supply travelers of all specialties, and finding any recruiter who was an specialty RN of any kind is exceedingly rare, so you are not going to speak the same language anyway. I worked for two of those three companies, and my recruiters knew nothing about the OR. Call is something most recruiters will need help understanding - and heck, call policies can vary dramatically between hospitals.
I don't think you will find and agency that has a preponderance of OR jobs. If they do specialize, they will have fewer hospital contracts. If you go with larger agencies, they will have more OR jobs.
OR travel was one of the best survivors of the economic downturn. Travel is back, and there are more jobs than travelers. So you should enjoy an easy time of getting assignments. CVOR especially.