If you do a fellowship and wish to practice HPM, your job opportunities will be the same as everyone else that did HPM fellowship (sans those occasional positions that specifically want IM/FM background -- often those are hybrid positions where the doc will also function within IM/FM realm, or just an ignorant creator of the listing).
If you wish to practice OBGYN and HPM, your best bet is to be at an academic center. You would likely be hired for a OBGYN position and could negotiate some FTE toward HPM duties (if they are open to it).
For example in my fellowship there were physiatrists, psychiatrists, hematologists, and pain medicine docs that would all rotate random weeks on our consult service as they were split between their primary field and small FTE worth of HPM duties. All of those folks did HPM fellowship.
What sort of career do you want?