I was asked to write up a quick summary of how to properly use the OGL to make content based off of WoTC content. This is accurate to the best of my understanding, but with the caveat that I'm not a lawyer, and I recommend everyone read the OGL before posting any derived works for sale! Please feel free to mention any corrections or missing important stuff here or ask any questions about OGL/Licensing in general here.

D&D is by far the most popular game out there, and it's a place ripe for developers and designers to cut their teeth and reach a bunch of people with their ideas. Making settings, monsters, classes and other content for D&D is likely the first place most people get a start on the content creation side of things.

However, when you make stuff for use with D&D, you have to follow the OGL or the Open Game License. This is a document that comes with a lot of D&D products these days that specifies exactly what is "Open Content" and what you're allowed to do with it. It's most commonly attached to the SRD or System Reference Document, which you can find here - http://media.wizards.com/2016/downloads/DND/SRD-OGL_V5.1.pdf - the first thing to know is that if it shows up in the SRD, you can use it in your project. You can quote it, you can copy it, edit it, whatever you like. Its "Open" by the terms of the license.

If you use anything in the SRD, you have to include the full 2 page OGL attached to the SRD. You also have to make it clear when and where you're using something from the SRD - if you use a creature or a stat block from the SRD, you have to include a note that it's reproduced under the OGL.

It seems complicated, but the basic breakdown is - if it's in the SRD, you can use it, so long as you include the OGL in its entirety and make a note wherever you use SRD content. Something like “Reproduced under the OGL” under the stat block or whatever is fine. So that sounds pretty good, but what can't you do?

Well, a lot. The OGL itself specifies the following are part of WotC's Brand and cannot be touched:

"The following items are designated Product Identity, as defined in Section 1(e) of the Open Game License Version 1.0a, and are subject to the conditions set forth in Section 7 of the OGL, and are not Open Content: Dungeons & Dragons, D&D, Player’s Handbook, Dungeon Master, Monster Manual, d20 System, Wizards of the Coast, d20 (when used as a trademark), Forgotten Realms, Faerûn, proper names (including those used in the names of spells or items), places, Underdark, Red Wizard of Thay, the City of Union, Heroic Domains of Ysgard, EverChanging Chaos of Limbo, Windswept Depths of Pandemonium, Infinite Layers of the Abyss, Tarterian Depths of Carceri, Gray Waste of Hades, Bleak Eternity of Gehenna, Nine Hells of Baator, Infernal Battlefield of Acheron, Clockwork Nirvana of Mechanus, Peaceable Kingdoms of Arcadia, Seven Mounting Heavens of Celestia, Twin Paradises of Bytopia, Blessed Fields of Elysium, Wilderness of the Beastlands, Olympian Glades of Arborea, Concordant Domain of the Outlands, Sigil, Lady of Pain, Book of Exalted Deeds, Book of Vile Darkness, beholder, gauth, carrion crawler, tanar’ri, baatezu, displacer beast, githyanki, githzerai, mind flayer, illithid, umber hulk, yuan-ti"

You can not reference, mention, use, replicate or modify anything to do with any of the above. Technically you can't even mention it in your text. Doing so violates the license and puts you in trouble with Wizards.

The other part to keep in mind is Section 7: "7. Use of Product Identity: You agree not to Use any Product Identity, including as an indication as to compatibility, except as expressly licensed in another, independent Agreement with the owner of each element of that Product Identity. You agree not to indicate compatibility or co-adaptability with any Trademark or Registered Trademark in conjunction with a work containing Open Game Content except as expressly licensed in another, independent Agreement with the owner of such Trademark or Registered Trademark."

What does this mean? Very simply, you can't say or imply that your game is for or compatible with D&D. I know this seems weird, and a bit backwards, but them's the breaks. You can still make the content, publish and sell it, but by the terms of the license, you can't say "This is a D&D 5th Edition Game". You CAN reference the SRD (By adding "System Reference Document 5.1 Copyright 2016, Wizards of the Coast” to your game description), you can say "Compatible with the world's most popular RPG system" or something similar, you just can't say "Dungeons and Dragons" or "D&D". You can also reference the OGL itself to describe what your creation is. Just don’t explicitly reference Dungeons and Dragons or any trademarks or elements of the Product Identity, and you should be clear.

You also can not reproduce any mechanics for character creation anywhere in any of your books - you can make new classes, new backgrounds, new spells, etc, but you can not tell people the rules for picking them, unless you're making up new ones somehow. The same goes for making monsters, stat blocks and so on - you can make new ones, just don't reproduce the rules for making them anywhere.

Muddying the waters somewhat is the Dungeon Masters Guild, which is a storefront that has some very different, much looser rules on producing content for D&D. You can find out more information on the DMG here -  http://dnd.wizards.com/articles/features/systems-reference-document-srd

Despite the somewhat confusing and overworked way the OGL works, once you get to grips with it, it's actually not as restrictive as it may feel, and it's perfectly ok for you to create content under it and post it here on Itch.