When are the cheapest flights from Oklahoma City, USA to Chongqing, China

I have a friend who is in there and she wants me to make an extended visit. I would need to fly one way as I would not have a definitive return date. I would plan on staying 4-6 months. When are the costs in general cheapest for flights from Oklahoma City, Oklahoma, USA? Would it be better to get a roundtrip flight with a return date 4 to 6 months in the future or purchase two one way tickets (is this even permitted). I have the proper visa for an extended stay so that is not an issue.