That is true - the OSI didn't coin it. But Bruce Perens of the OSI trademarked Open Source with the definition as the rules to apply it.

I think that basically puts to rest the question of who "owns" Open Source.

Bruce attempted to register a certification mark (a type of trademark), but the application lapsed in 1999 after OSI discovered that there was virtually zero chance of registering the term "open source" as a mark, as the term is too descriptive.

OSI has "OSI Certified" as a registered trademark (a certification mark).