Summer: Its hot (sometimes over 110 degress F) its humid and sticky. Most wear shorts and short sleved shirts.
Winter: It can be below freezing with mountains of snow so come prepared! The weather changes constantly and isn't really predictable so be ready for anythiing. Some things that are popular currently are; abercrombie, hollister, american eagle, flip-flops, gap, punk rock and turquoise jewerly. On the whole pretty much anything goes.
Keep an eye on the weather forcast for Oklahoma for several days before your trip. And then pack a few items for the opposite weather. There is a saying in Oklahoma that if you don't like the weather, wait another day. It will change... 70 degree temp. days are all too often followed by 30 degree days. And don't let that brightly shining sun fool you. I was continually dressed inappropriately for the weather while growing up there. The day before would be cold, so I would bundle up that morning, only to discover by noon, that a warm front had come through. And vice versa.
NOTE: If you are travelling to Tulsa in the summer, the weather will be hot and sticky no matter what. It will not change until late September.