Well, remember that the big social event in "Oklahoma!" is the barn-raising/fund-raiser/dance that everyone is getting ready for. So the costumes should reflect that. The first part of the show is early in the day. The clothes should be average "working" clothes for the period. As the show progresses and the day goes on, everyone is getting ready for the big event, so they would be wearing more colorful, party, or special ocation clothes. Not your everyday attire. The "chorus" of visitors that stop over would likely be wearing traveling clothes or over-garments (to keep the dust off of them on the trip), etc.
I think you need to pick apart the action in the show, and figure out not just what "folks were wearing in the Midwest" back at the turn of the last century, but find the appropriate clothing for the actual events at hand.
"Functional as well as ornamental..."
Hope that helps.
Creatively, you'll have to decide on your own if you're going for realism or a more stylized interpretation overall.
"Jaws is the Citizen Kane of movies."
blocked: logan2, Diamonds3, Hamilton22