No, "open source" was not in common use at the time that OSI decided to use it. It's certainly not been used to mean anything except roughly OSI's definition until approximately two years ago, when Redis Labs decided to push the Commons Clause as being open source.
It was in use, both in software and in the world at large (especially the intelligence community). I suppose how it depends how you define "common" but "people use the term in official announcements and assume everyone knows what they mean" seems common enough to me.
OK, now find me regular usage of "open source", applied to software, by more than one person trying to sell something, as meaning something different from the OSI definition between approx 2000 and 2017.
You haven't shown that "open source" was in common use at the time OSI decided to use it (1988 is earlier than 1996), nor that it's been in common use as meaning something else when applied to software since then. One company trying to sell something doesn't make it common use, either - otherwise I could claim that my water is from a "free source" and declare that for all eternity referring to free source must mean it comes from my specific river.
Ah, thank you. Either way, one company using the term once before that doesn't mean that it is a phrase in common usage. It also doesn't mean that the term hasn't changed to mean something else in the eyes of hundreds of thousands of developers since them, and using it to mean its plain meaning will only ever cause confusion, and potentially result in people doing things you didn't want with the source code.
Epic, Microsoft, IBM, and hundreds of other companies, big and small, have been careful to avoid this issue specifically because there is an existing definition and it causes confusion.
