When I've encountered people afar wearing OU gear, I've pretty much learned to wait for them to say something to me first. I used to ask questions like, "Do you follow the Sooners?" or "Are you a Sooner fan?" only to get a response such as, "No, my wife bought this cap for me".
One time, a Cortez Kennedy-looking guy was walking towards me in the concourse at the St. Louis airport. He was wearing an OU cap and I just knew this guy loved the Sooners. I got face to face in passing and said (not yelled) "Boomer!". The guy looked pissed off at me. Gave me the old WTF? look. That's when I knew it was time to keep to myself from now on.
Kinda off on a tangent here, I also used to speak to people wearing other college gear. One time I was in Alamogordo, NM at a convenience store. These two guys were wearing Texas Tech stuff. I said to one of them, "Wow. You guys hired one a heck of a Basketball coach. I bet you're excited." I could tell by the confused response that he was clueless about who I was talking about. Bobby Knight had been hired the week before.