I'm not 100% sure, but I don't think the O'Reilly came up with the concept of drinking kool-aid as meaning buying into an ostensibly outlandish theory or argument. I can recall that being a part of popular vernacular since before the O'Reilly Factor debuted.
Interestingly enough, the Guyana cultists didn't actually drink Kool-Aid, they drank Flavor-Aid. Kool-Aid is just the more recognizable brand.
