Can I sue onstar for letting someone register my car without my permission

I changed the practice area to litigation, this is not an assault question. Sure, you can sue anyone you like if you pay an attorney to file the lawsuit. However, I would not expect to be able to find any attorney to take such a case on a contingency fee though (you don't pay unless you win the lawsuit).  Attorneys typically never take a case on a "contingency" unless there is provable, measurable injury, where there are provable, measurable damages significant enough to pay the attorney and the harmed party. But if you are willing to pay an attorney a retainer fee that is thousands of dollars to begin working on filing the lawsuit, absolutely - you can sue them. Good luck.