Something I feel is overlooked a lot is how Chaosium’s Basic Roleplaying (BRP), and through it, such games as Call of Cthulhu, are accessible via the OGL.
To understand that, we need to look at what BRP is used in. BRP originated as the rules for the RuneQuest RPG in 1978, soon became a standalone ruleset of its own in 1980, and was then used for other Chaosium products such as Call of Cthulhu, Stormbringer, and Elfquest.
After four editions of RuneQuest, the rights to Glorantha (the world the game is set in) material left Chaosium with Greg Stafford and passed to his new company, Issaries. In 2011, Mongoose Publishing licensed the rights from Issaries in order to create the fifth edition, commonly refered to as “Mongoose RuneQuest”, or “MRQ”.
MRQ used a close variant on the same rules, but rewritten from scratch to avoid copyright issues (as Chaosium still held the copyright on the original texts, even if they no longer had the trademark or the rights to the setting.), and was released under the Open Game License, the license Wizards of the Coast created in order to “open source” the third edition of Dungeons and Dragons, along with a System Reference Document (SRD) on their own website.
This “Mongoose RuneQuest SRD” pretty much covers the majority of BRP rules at the time, with a couple of changes. Mongoose went on to create Runequest II, and Legend, a game using the same OGL ruleset, after their rights to use the RuneQuest name and Glorantha setting lapsed.
Issaries then licensed the rights for a 6th edition of RuneQuest to The Design Mechanism.
Since that time, the RuneQuest rights have gone full circle, with Issaries selling the rights to Moon Design, who then merged with Chaosium after Greg Stafford and Sandy Petersen re-took control of the latter.
However, that original OGL Mongoose edition of RuneQuest plus it’s derivatives (RuneQuest II and Legend) still open the ruleset up via the OGL. It’s suggested that anyone wanting to use those rules do so via Legend, rather than RuneQuest, in order to avoid trademark issues. And that’s just what Cakebread and Walton did for their Renaissance rules.
It’s important to remember that rules cannot be copyrighted, only the text expressing them. So it is usually legal to rewrite any rules in your own words, just so long as you keep it suitably different to the original, and avoid using any trademarked terms or other unauthorized Intellectual Property (which, under the OGL, may be more than just Trademarks). Therefore, with a bit of work, you can add any “missing” rules such as Call of Cthulhu‘s Sanity system to one of the OGL-ed BRP derivatives, and roll your own set of, say Call of Cthulhu-compatible rules – or release a product designed to work with the original. As with any OGL products, of course, you are not legally able to mention the original game your product is designed to replace or work with (unless you can somehow get permission from the current owners), so you have to get creative figuring out how to indicate compatibility!
As usual, this document is not intended to convey legal advice, consult a lawyer if you are not 100% certain of your rights.