Why do Oklahoma fans say boomer sooner?

Could you explain the origin and significance behind the Oklahoma fans' chant, "Boomer Sooner"? I've heard it at games and it seems to be a rallying cry for the team, but I'm not entirely clear on its meaning or how it came to be associated with Oklahoma football. Is it a historical reference? A play on words? Or something else entirely? I'd appreciate any insight you can provide.