The red, white, and blue represents the United States. The oil-colored square represents oil (duh?). The square is also used to make the blue area resemble the shape of Oklahoma. The white circle and the blue field represents the sun shining.
The current Oklahoma flag has an Indian shield and peace pipe, at least, I think that's what they are. It also has the name of the state at the bottom. Yuck. What if the flag was viewed from the other side? "AMOHALKO" drow eht dna ngised detacilpmoc a evah dluow tI