As cliquesh said, OMM is sorta a niche area of medicine. Most insurance will pay for it in my experience though. Heck, even the EMR at the site I'm currently at has OMM findings that you can add to your physical examination on the progress note.
But also as cliquesh said, most OMM doc's are Cash only. The one office I was exposed to last year required cash, but offered to help patients seek reimbursement from their health insurance companies.
Most highly proficient OMM specialists I've met do extremely well. Some even making $500K or more per year. A few of the ones who seem to focus more on the fringe elements of OMM such as Craniosacral therapy etc. seem to struggle more (again based on what I've seen).
OMM also helps some DO's land some pretty sweet Sports Med gigs too. One of the dean's at SOMA (now the ATSU president) was the team physician for Phoenix's NBA and WNBA teams until last year or so.