I live in Oklahoma too - up in Central Oklahoma. The horse flies got really bad a couple of years ago. So bad, that in spite of fly spray on the horses, the poor horses had bloody shoulders and necks. Put out the sulfur salt blocks, and the horse flies disappeared. We now put out sulfur blocks as soon as the weather starts warming up, and haven't had a problem with horse flies and other biting flies since. Oh...they are still around, but they aren't bothering the horses any.
Sadly, it doesn't do a thing for the other flies. The black flies that swarm all over the horses and drive them nuts. Fortunately, the Pyrhanna (spelling?) fly spray/wipe works quite well for those, but we have to spray the horses every couple of days.
If you don't want to use a commercial spray, there was someone on this forum that recommended neem oil. I've heard of it before, but haven't tried it. I wonder if it's similar to tea tree oil?
Plain Old Dee, horses Dancer and Rain I believe in dragons, unicorns, good men and other mythical creatures!