> No, Open Source and Free Software are two names for essentially the same thing.

This is not substantially true, which is why I assume you've added "essentially" in here. Open Source is Free Software, because anybody can take it and make it anything they want as long as they comply with the minimal license terms. Open Source can be proprietary, too, if somebody takes it, complies with the minimal license terms, and makes it proprietary.

> This is not substantially true, which is why I assume you've added "essentially" in here.

No, it is. The OSI Open Source definition and the FSF Free Software definition are framed differently but require substantially the same things, and for virtually every license on which both have expressed an opinion, they have cone to the same conclusion as to whether it meets each organization’s requirements.

Free Software does not require a license that prevents proprietary re-licensing, that is an additional separate concern beyond the Free Software definition (Copyleft); the FSF generally prefers copyleft licenses, but recognizes non-copyleft licenses as Free Software licenses.

You seem to under the mistaken impression that copyleft is a requirement to meet the Free Software definition, but that has never been the case.