not much you can do but a lesson to learn: never go to OON

OON (Out Of Network) means your insurance company has no contract with the health care providers (physicians or facilities), what they bill is what they get. Usually it is stated in your benefit how much (usually % of billed amount) your insurance company will cover for your OON bill, in your case, 50%, I would say your insurance company is generous.

I would say your Dr. is unethical, he took advantage of your trust. Physicians should make their bast effort to use in network providers, it usually is stated in contract. You may try to complain to your insurance company.