At this point it's probably easier to just eschew the OGL and publish for Dungeons and Dragons by name than it is to actually publish something that would be compatible with the Dungeons and Dragons OGL-enabled restatements, if you're not interested in throwing your hat in with any particular version.
Making compatible products with an existing game without an license is a legal gray area. Courts have not be a definitive ruling. Go to one lawyer you get answer X, and goto another you get Y. For a more concrete example see the differences between the opinion of David Kenzer of KenzerCo and Clark Peterson of Necromancer Games. Both are lawyer both have intellectual property experience. David believes that the law favor him publishing compatible products without a license, Clark believes it doesn't.
The Open Gaming License however is explicit permission to use copyrighted material with the condition that give the same right with what we create from it. Luckily for us the terms and much of the stats we need to make products compatible with older edition is found in the d20 SRD.
What it boils down to is risk. Whether you want to risk being sued by Wizards for copyright infringement. Granted it not going to happen right off the bat. Likely you will get a cease and desist letter first. But in either case your business plan, sense of well-being, and most importantly fun will be trashed.
Unless you are one of the lucky few who is a intellectual property lawyer, none of us are experienced enough in law to decide what is the exact risk we are assuming.
For people who are starting out in life, in their 20s, don't have much assets or a family to support then the risk of making compatible product is different then somebody in their 40s, has a family, a house, and some assets.
Also the risk is different depending on how serious you are about making money off of publishing professionally. If you are going invest some money and time then your view of the risk involved is going to different.
What the OGL does is reduce the risk. By properly following the license you nearly eliminate the chance that Wizards is going to send even a cease and desist letter.
The way it works is as follows
- You are using the d20 SRD to write a product compatible with OD&D.
- You ignore anything in the d20 SRD that is not found in OD&D or it's supplements.
- Anything in OD&D and the supplements that is not found in the d20 SRD you don't use.
Now most people including myself will start substituting numbers from OD&D for the d20 numbers for things like AC, HD, etc. You are assuming more risk in theory but given the customization options under the d20 SRD it would be hard for Wizards to make a case out of it in my opinion.
The line starts being crossed when you copy things like the to Hit table that can't be derived mathematically. Of course copying any text directly from an older edition is definitely over the line and sooner or later you will get missive from Wizards.
Then to make it crazier different products have different risk. Stuff like the Wild North is very low (as regards to OD&D) all I was using was class,level, or HD and names of generic magic items. The risk with a adventure where fuller stats are needed is probably a little higher. More risk with a rule supplement like a book of classes or a monster book. Then finally the highest is making a direct clone or variant of an older edition.
I am not a lawyer, any legal advice I have is not worth anything. But the OGL is meant to be used by ordinary people to make products commercially . So I feel pretty safe in using the OGL to make products compatible with OD&D.
It a little harder of course because I have to come up with original alternative to the stuff in OD&D that not in the d20 SRD. But in the end it will mean product in your hands, satisfaction on my part that people are having fun with the stuff I create, and yes having a little money to show for all the time I put into my favorite hobby.