Is a seller of a car still respossible for property damage caused by buy until buyer re-register s car?

If its an arms length transaction, and the car was really sold, then there generally is no liability for damages caused by a car which happened after the sale.  Registration is not determinative because in Oklahoma, registration does not mean ownership; and without ownership, there is no right of control; and if there is no right of control, there can be no liability for the acts of others.  In any event, you should advise your insurance company of the accident and the sale; and let them handle it for you.