As per the Help Center, this site uses two resources to guide its definition of whether something is "open source" or not:
These aren't a perfect match for the OGL because they are both somewhat biased towards free software rather than free content in general, but they can still probably act as a good starting point as to whether we think the OGL is "open source" or not.
FSF's Four Freedoms
Freedom 0: The freedom to run the program as you wish, for any purpose
Translating this to something like "the freedom to use the content as you wish, for any purpose", this freedom would seem to me to be satisfied by the OGL Clause 4:
the Contributors grant You a perpetual, worldwide, royalty-free, non-exclusive license with the exact terms of this License to Use, the Open Game Content.
with no conditions attached to that.
Freedom 1: The freedom to study how the program works, and change it so it does your computing as you wish
This freedom is primarily based around the distinction between source and binaries for software; that's a distinction which isn't really relevant for tabletop gaming content.
Freedom 2: The freedom to redistribute copies so you can help others
The OGL's definition of "Use" is (from Clause 1):
"Use", "Used" or "Using" means to use, Distribute, copy [...] Open Game Content.
That freedom would seem to be satisfied.
Freedom 3: The freedom to distribute copies of your modified versions to others
Again from the OGL's definition of "Use":
"Use", "Used" or "Using" means to [...] edit, format, modify, translate and otherwise create Derivative Material of Open Game Content.
That pretty explicitly allows creation of modified versions.
Open Source Definition
Being slightly more terse here:
- Free Redistribution: there are no restrictions in the OGL about OGL content being distributed with non-OGL content; in fact, this is very common for OGL content.
- Source Code: as per FSF Freedom 1, not really relevant.
- Derived Works: as per FSF Freedom 4.
- Integrity of The Author's Source Code: not really relevant as this is again about a source/binary distinction.
- No Discrimination Against Persons or Groups: nothing of this sort mentioned in the OGL.
- No Discrimination Against Fields of Endeavor: nothing of this sort mentioned in the OGL.
- Distribution of License: no mention of additional licenses in the OGL.
- License Must Not Be Specific to a Product: no mention of specific products in the OGL.
- License Must Not Restrict Other Software: even when "translated" to "License Must Not Restrict Other Content", there are no restrictions of this kind in the OGL.
- License Must Be Technology-Neutral: no mention of anything of this kind in the OGL.
So, is the OGL "Open Source"?
From the above analysis, the prima facie answer would be "yes". However, one major issue lurks: as noted on the FSF's page, it is absolutely crucial that any open source license is irrevocable, as a license is in many ways not worth anything if all your rights can be removed tomorrow. Is the OGL irrevocable?
The OGL grants "a perpetual, worldwide, royalty-free, non-exclusive license" (Clause 4 again). I'm aware there has been an enormous amount of mostly amateur discussion specifically around the OGL as to whether a perpetual license is in fact irrevocable; that would seem to be a question which we'd only get a real answer on when a court rules in a specific jurisdiction. However, it remains a fact that the leaked draft of the OGL 1.1 did in fact try to revoke the OGL 1.0a, and whatever Wizards of the Coast may be, one thing they certainly do have is some good lawyers - if those lawyers believe it can be revoked, I'd say there's enough uncertainty about it that it could well be.
Therefore my conclusion would be that the OGL is not an open source license.