No. You DO NOT have permission to use Delta Green, either the property or the trademark, in your publication or in promotional materials without the express written permission of the Delta Green Partnership.
The Open Gaming License in the Agent’s Handbook (page 190) explicitly covers only a selection of the rules of that book, some of which were derived from other OGL publications. It covers absolutely nothing else.
Here is a summary of the book’s Open Game Content, which is available for use under the terms of the OGL: Game mechanics on pages 14–22, 28–36, 42–47, 50–63, 66–75, 78–79, and 84–85, except for material designated as Product Identity.
Here is a summary of the book’s Product Identity, which is not covered by the OGL and is not available for use, no matter what page on which it may be found: The intellectual property known as Delta Green; all trademarks, proper names, artwork, and trade dress; and all text not explicitly identified as Open Game Content.
Those summaries are not the entirety of the Open Gaming License. Read the license itself, in detail, before you use any material from the Agent’s Handbook in your own publication. If you find anything in the OGL confusing, consult with someone who can help you understand it.
Can I publish Delta Green material for free on my personal website?
We encourage Delta Green fans to make material available online for the edification and enlightement of others. You are free to publish such material as long as there’s no cost for anyone else to use it.
In order to protect Delta Green operational security and the intellectual property rights of our publishing venture, we require that the following notice be appended to each web page that makes use of Delta Green characters and settings:
Published by arrangement with the Delta Green Partnership. The intellectual property known as Delta Green is ™ and © the Delta Green Partnership, who has licensed its use here. The contents of this document are ©<fill in author’s name>, excepting those elements that are components of the Delta Green intellectual property.
If you have any questions about whether or not you need to apply this notice to a particular page, please contact Shane Ivey, email@example.com.
This license applies only to free, noncommercial works. Any paid or commercial product that uses Delta Green or Delta Green-associated property must be separately and specifically licensed from the Delta Green Partnership.
I summon and bind the power of Wikipedia!
What’s a role-playing game?
Wikipedia, speak again! (Note: Delta Green is usually played as a tabletop RPG.)
The original sourcebook, Delta Green, remains the most essential resource for running a Delta Green game. Delta Green: Countdown, Delta Green: Eyes Only, and Delta Green: Targets of Opportunity widely expand and enhance the setting. Those Delta Green products were written for Chaosium’s Call of Cthulhu role-playing game, so understanding that game’s rules and settings will enhance your use and understanding of Delta Green.
In Delta Green, nothing is canon. Consider the novels, short stories, and online articles the way you would consider logs or journals of other players’ games, as examples of stories or games which other players have experienced. All players are encouraged to use what materials are appropriate and most effective for their games, whatever the source. Use what works; do what you will with the rest.
Different writers have different styles, attitudes, and ways of presenting Lovecraftian horror. In addition to Lovecraft himself, Delta Green tends to associate itself most closely with Ramsey Campbell and Robert Chambers, and least closely, in many ways, with August Derleth and Brian Lumley.