I was afraid somebody would ask me that.
Actually, I'm a "rookie" OU fan since my son is only a freshman there and I am a graduate of the Univ. of Texas so I never took much interest in any OU sayings, etc.
If you want to find out more you can visit the OU athletics website soonersports.com and I think there is a section on OU traditions that better explains it. I am still learning but I know that some OU fans say Boomer Sooner when greeting other OU fans. I think it has something to do with some settlers starting the "land rush" in Oklahoma territory a little early. I know that the nickname Sooners has to do with that. In Texas, they are referred to as "land thieves."
Sorry I can't be of more help but when I get the time I am going to learn more about OU history and traditions.
Maybe some native Oklahomans will see this post and answer your question better.
Ann