Could Restylane Have Caused an Osteoma on my Skull?

As far as I know, there is no connection between restylane injections and the development of osteomas. Restylane itself has no calcium in it. Even if you were injected with Radiesse (which contains calcium hydroxyapatite, a component of bone) in the nasolabial folds, it would be extraordinarily unlikely that the filler injection could have caused an osteoma at a distant site. I hope you have that osteoma taken care of soon so you are no longer in pain.

This would be a total coincidence in my opinion. It does not make sense either medically or biologically.
During my years in practice I have had a few patients present with osteomas. They are harmless lesions and in fact many pathologists do not regard them as true neoplasms but as a reactionary process (the body is reacting to something and laying down bone). They nearly always occur on the skull like yours. You might shake your family tree to see whether any intestinal polyps, epidermoid cysts (sometimes erroneously called sebaceous cysts) or fibromas pop out. If so you might have Gardners Syndrome of which osteomas are a key part.
I referred them to a plastic surgeon and the problem was corrected with surgery. Sometimes the plastic surgeon justs buffs the osteoma down. ( You might ask the plastic surgeons on this worthy site how exactly they treat this problem. An Erbium Yag laser approach can also be taken.
Good luck.