OSI couldn't trademark "Open Source" and the common usage of the term (source available) has steered away from the OSI definition.
Common usage has matched the OSI definition for decades. Even Wikipedia acknowledges it [0]. There are always some people who are confused about the terms, sure, but those usually come simply from lack of exposure and experience around FLOSS communities. Trademarks have nothing to do with it; it's not OSI that owns the term, but the community that uses it and widely agrees with OSI's definition.
[0] https://en.wikipedia.org/wiki/Open-source_software#Definitio...
the obvious meaning for the expression “open source software” is “You can look at the source code.” This is a much weaker criterion than free software; it includes free software, but also some proprietary programs, including Xv, and Qt under its original license (before the QPL).
That obvious meaning for “open source” is not the meaning that its advocates intend. The result is that most people misunderstand what those advocates are advocating. Here is how writer Neal Stephenson defined “open source”:
Linux is “open source” software meaning, simply, that anyone can get copies of its source code files."