my brother is 18 his girlfriend is 16 can he get trouble for being w her

In Oklahoma, the statutory age of consent is 16. If the 18 year old and the 16 year old are involved in a consensual sexual act, the 18 year old would NOT be found in violation of the sexual offense statutes.

It should be noted that the 16 year old is still a minor child under the parents authority until the age of 18. This means, the parents legally have the right to restrict or prohibit the minor's activites and contact with anyone.