(usual disclaimer that I'm not a professional lawyer and that you should consult one)
You're quite right to question the PRD as being 100% open content, as there are indeed some PI terms used within it. The PRD's Open Content declaration only specifies that rules included within are open (in similar wording to many printed products that state the same thing), at no point does it actually state the entire text of PRD is Open Content. In addition, PI falls under the implicit terms of the OGL as being protected in section 1(e) of the License. In that way, the PRD cannot be considered as 100% Open Content in the same way as the d20 SRD can.
Your best bet is to pick out those terms, and email Paizo directly to query whether it is okay to use them under the OGL.
One notable example is the Prestige Class "Pathfinder Chronicler", which is a specific term used within Paizo's Golarion setting background and not a generic name such as "Loremaster", "Wizard", or "Fighter", as well as containing the term "Pathfinder" which is protected as PI as the product name itself. In this example, you can create a prestige class in an OGL product using the rules of the Pathfinder Chronicler (or simply reproduce the class itself), but would have to call it something different. To borrow a little from Mxyzplk's answer, you could probably call it "PF Chronicler" ;)
Note that the advice I'm giving is based upon the interpretation of Paizo's OGC declaration, not the intent. It may well be that they intend all terms within the PRD to be Open, but they have not specifically said that in their declaration. Obtaining permission/clarification from Paizo is a simple matter of sending an email, so there's really no reason not to ask them to be 100% sure on the matter. Assuming it is okay because other products have used those terms (and they have) is not good practice - it only means you'll also be in violation if the OGL if they are.
Paizo contact emails can be found at http://paizo.com/paizo/about/contact - my guess is the Licensing Coordinator would be a good place to start asking.
Yes, originally expected in 2015, 5e OGL and SRD arrived on Jan 2016.
OGL is now embedded in the SRD, available here:
http://dnd.wizards.com/articles/features/systems-reference-document-srd
5e OGL is essentially the same with 3r OGL word for word (see below).
Like 3r SRD (and unlike 4e's), it contains de-flavored basic player and DM resources that you can use to create other OGL materials.
Please keep in mind that Basic Rules is not SRD5, and SRD5 is not 3r SRD.
For example, SRD 5.0 includes experience table (under "Beyond 1st Level", page 56), but simplified all classes to their archetype, and contains only one feat.
Wizards of the Coast has also created a new online market, Dungeon Masters Guild, that allows you to extend proprietary materials such as Forgotten Realms and sell them, sharing the profit with Wizards.
Otherwise, what you can do with the SRD and what you cannot do remains the same as 3r.
You can still distribute (and sell) your SRD based OGL materials, outside the guild, as long as the materials does not use "Product Identities" and does not violate other copyrights.
Differences between 5e OGL with 3r OGL:
- Term 7, last sentence, rights are now retained for Product, not Product Identity.
- Term 15 now refers to
System Reference Document 5.0 instead of System Reference Document. The years and authors has also been updated.
- Legal Information, Product Identity list now includes "Underdark", and also refers to new SRD 5.0.
Historically, the plan was an announcement @ 2014 fall and release in 2015.
Here is an official post about it:
We want to ensure that the quality of anything D&D fans create is as high as possible.
Basic D&D is aimed at new players ... not for material that you want to share broadly.
It'll take time for everyone to absorb the rules and how they all interact.
While the details are still in flux, we can say that we plan to announce the details of our plans sometime this fall. After that announcement, we plan on launching our program in early 2015.
This matches some other third party comments, which hints that they want to do something different from OGL:
Mearls' plan for D&D is largely the same goals that were created for the OGL.
The difference was that the OGL (assumed) that publishers would create a better game by cooperating through iterative design;
instead, authors were motivated to ignore each others' innovations and recreate the same rules so that they were paid additional cents per word.
It may be related that Wizards has not granted any translation license yet. Perhaps the developers are focusing on core rulebooks and neglected licenses. The real reason may be unrelated, of course.
As far as I know, as of 2016 Aug, there is still no official 5e translation.
Everyone may translate and release SRD5 under the terms of OGL, however, which is what Hobby Japan do.
(Update 2017 March: Finally, after 3 years, Gale Force Nine will translate D&D 5e into multiple languages, starting with French, German, Italian, Japanese, Spanish, Polish, and Portuguese. This stands in contrast with previous editions where a local publisher would buy the right to translate and distribute the local language.)
Best Answer
It appears that you cannot.
The issue here is not copyright. Titles and short phrases generally aren't protected by copyright — there is trademark law for that. Dungeons & Dragons certainly is a trademark, but even that isn't really the point here — you can probably use trademarks in a nominative sense even without special permission (hello, Boston Marathon). The important thing is: you want to use 5E stuff beyond just the name, and the OGL — the Open Game License — is what lets you do that. If you want use that license, you have to do what it says. The relevant section is:
... and in the 5E SRD, the phrase "Dungeons & Dragons" is clearly designated as "Product Identity" — in fact, it's the first thing listed. So, unless you have a separate agreement, it looks like the answer is no — but, of course, keep in mind that I am not a lawyer, and this is just the interpretation of some guy on the Internet.
Note that the wording specifically forbids use of this "Product Identity" — including "Dungeons & Dragons" — in conjunction with a work, not just "in the work itself". This would appear to cover the website situation. This is probably more restrictive than normal trademark rules — but that's basically part of the price you pay in order to get the benefits the OGL grants you.
As with "Marathon Monday" or "The Big Game" in the sports world, you'll probably need to find an alternate phrase which your audience will understand to indicate compatibility without violating the license.