The writings of H.P. Lovecraft, Robert E. Howard, and Michael Moorcock contributed significantly to the concept of an OD&D game world. Fritz Leiber's "World of Newhon", while equally influential, wasn't detailed until AD&D's "Deities & Demigods" (1980).
The Lovecraftian Mythos:
See Zenopus Archives for a comprehensive survey of "The Cthulhu Mythos in D&D in the 1970s".
In their original "Known World" setting, Lawrence Schick and Tom Moldvay imagined that "in every land there would be hidden cults that worshiped Lovecraftian Elder Gods."
The Hyborian Age:
The Hyborian Age, Marvel Comics version.
Howard's Conan stories were set during a pre-cataclysmic Hyborian Age, circa 10,000 BC. These were later adapted by Marvel Comics, becoming part of the Marvel Universe.
The OD&D game world might have likewise had a Hyborian Age. Players desiring to adventure in Howard's fictional setting would need to travel into the distant past.
The Melniboné Story Line:
The initial treatment of "Law" and "Chaos" was inspired by Michael Moorcock’s treatment of good and evil in his "Elric" and other fantasy books written prior to 1970.
Gary Gygax, interview
The Elric stories were originally conceived by Moorcock to have taken place on an alternate earth, in an even more distant past.
Gary Gygax referred to the setting for the Battle of Brown Hills as "a mythical continent", predating that of the Great Kingdom. I like to consider this scenario part of OD&D prehistory.
As the cosmic conflict between Law and Chaos faded in significance, so too might the conflict have represented a primordial one in the OD&D game world.