I can't see how this is as easy:
I think that "PI" is basically everything covered by regular copyright, and about 90% of what you see as "OGC" is stuff that's not protected by copyright anyway. (If I'm wrong about this, I'd be interested in examples)
Uhhh. yeah. I'm not an IP attorney, and this isn't legal advice, However here is my take on IP.
PI and regular copyright are not even close to the same. You have to define
Product Identity in the OGL, and generally the more vague you are with your definition of PI, the more vague the intrepretation of your PI becomes. Meaning... if you define a whole chapter as PI, and the court finds any elements of that chapter that don't constitute PI, they could easily throw out the whole chapter. Not a good place to be.
Games are defined by rules and mechanics, both of which are not subject to copyright or PI (unless your rules are in fact unique and original which most aren't, and remember any derivative works based on previously existing games do not constitute PI). The Artistic and Intrepretive elements of the game that do comprise PI create a theme, a mood, or a unique playing environment that is not featured in previously existing games.
Examples of PI include setting unique weapons and equipment, thusly;
Mygamian Serrated Long Scimitar 2d6+2
Mygamian Wizaldry Helm - A helm any character can wear that grants one additional spell per day per character level...
and so forth... You have to define this as PI in the OGL, that way, if someone does want to use the OGL, they know they can't include your
Mygamian Serrated Long Scimitar in their publications or rules. Setting unique features such as calenders, timelines, notable personalities, locales, geography, weather, document layout and style can all constitute PI.
Artworks are generally protected under copyright, and you should always obtain a written agreement specifying your ownership rights or right to use the artwork in your gaming publication. Most OGL's clearly define which chapters or subject headings constitute PI and are unavailable for reuse by a 3rd party.
Then there are Trademarks...
A trademark includes any word, name, symbol, or device, or any combination, used, or intended to be used, in commerce to identify and distinguish the goods of one manufacturer or seller from goods manufactured or sold by others, and to indicate the source of the goods. In short, a trademark is a brand name. Trademarks currently cost $325 and can be filed
Here to protect your Product Identity.
References
United States Patent And Trademark Office
http://www.uspto.gov/index.html