I don't know. I know the axiom is that if you're going to go into family practice or sports medicine that you'd be dumb not to learn it well. It's an unwritten rule that it's both sought after by patients and can increase your income. The great thing is that because it's taught in school, there really is no reason to specialize in it. Any DO can do it, although the AOA does have a specialty college for "neuromuscular medicine" where people do a 1-year fellowship to get Jedi-like good at it. They're supposedly competitive (yeah, I know). The truly good OMT practitioners are, if nothing else, sort of amazing to watch. They're fast and provide good affect on target, which is a problem for the uninitiated (students).
I also know some OMT-practicing DOs do well in the pain arena: complicated patients who are intractable to traditional treatments. Placebo or not, apparently they have a good success rate in getting people to accept their pain and work through it. Patients come in a few times a month for manipulation, plus OMT focuses on self-treatment and "empowerment. " Patients eat that stuff up and most docs will tell you they’re able to get them off stronger meds, so long as there isn’t preexisting dependence and a desire to change.
If you live in a DO heavy area, it's like an alternate universe. I grew up in an area with a decent saturation, but nothing like what I've seen where I'm at now. They honestly equal or outnumber the MDs. People in the area are hyper-aware of their existence and seek them out for all the marketing mantra we hear on here all the time.