Can a father get custody of a child in oklahoma even if he was never married to the mother?

The mother wont allow the father to have much visitation with the son. Its more of her bitterness toward the father than anything. He works alot and when he can he contacts her to see his son but either she says he cant or verbally aults him when he does come to see his son. The father pays her more than what is normal support for his salary but still cannot see his son when he can. She stays at home with the baby and is able to do that because of his financial support.....but still cant see his son when he wants. Various family members have stated that they question her care of the child. The mother doesnt allow much visitation with the father's parents.