Christine Peterson claims to have invented the term in 1998.
https://opensource.com/article/18/2/coining-term-open-source...
But at least this much is not true - we can see it being used, in context, at least as far back as 1996. I think people involved in this project at the time say it was a common term back then as well, but I can't find a reference for that now.
http://www.xent.com/FoRK-archive/fall96/0269.html
And they certainly don't own it - they wrote a whole article on their own website still available about how they don't own it!
https://opensource.org/pressreleases/certified-open-source.p...