It does pay, but it's a difficult specialty to build up a strong client base, from what I know.
Traditionally an MD radiologist has read the scans that oral radiologists would be needed for, and since few hospitals staff oral radiologists, most scans would indeed be read by a radiologist.
Some oral radiologists are not affiliated with hospitals, but with stand-alone imaging centers. Once again, their initial focus has to be on how to get referrals to them, rather than other imaging centers or hospitals.
The oral radiologist at my school has recently taken his company public and is having HUGE success with new technologies that he's developed. Go here to check out some amazing images from his scanning software and hardware:
http://www.clear-scan.com/
So all in all, money can surely be made, but it's going to require a lot more effort than dentist plunking down somewhere and setting up shop. The question will remain for some time, "why should I send my patients to an oral radiologist rather than a radiologist or whoever my pathologist recommends?"