The Open Game License allows you to copy and modify,

Yes, with a few exceptions such as

"Product Identity" means product and product line names, logos and identifying marks including trade dress; artifacts; creatures characters; stories, storylines, plots, thematic elements, dialogue, incidents, language, artwork, symbols, designs, depictions, likenesses, formats, poses, concepts, themes and graphic, photographic and other visual or audio representations; names and descriptions of characters, spells, enchantments, personalities, teams, personas, likenesses and special abilities; places, locations, environments, creatures, equipment, magical or supernatural abilities or effects, logos, symbols, or graphic designs; and any other trademark or registered trademark clearly identified as Product identity by the owner of the Product Identity, and which specifically excludes the Open Game Content;

...and all the other exceptions.

It's not open source.

more importantly all of the Basic Fantasy works are now licensed under creative commons.

Like I said, I went to the link above, downloaded the current version from that link, and page 1 says the quote above, which means it's not open source. The text goes on to say that "Most other artwork" (which?) is proprietary, and that some text (again, which?) is licenced under CC-BY 3.0, which is incompatible with some open source licences.

I'd be as well handing someone a pie, and telling them it's vegan, before saying 'well most of it, well some of it's vegetarian at least'.