I have 2 paid tickets to see LaTraviata on Dec15, 2018 at King of P The King of Prussia Theatre says they are sold out of reserved seats. Is it possible to use those opera tickets (and get reserved seats and buy a 3rd ticket) at the Oaks Theatre?

I would call the theater. They should know ticketing policy of the company that sold the tickets

Oaks is just a movie theater. They have no reservation on seats unless it's a sneak preview. I would bet your King of Prussua tickets won't transfer anywhere. Sorry.