not many people here are likely to be familiar with the oahspe (indeed, i had to wiki it) so let me rephrase the question using examples more generally at hand: say the book of mormon.
now, i can point out some historical problems with it, but i can do the same with the bible too. neither is
quite as old as it claims to be. but what's to say that joseph smith was not a divinely inspired prophet?
once we hammer out this slightly more obvious example, perhaps we can go on to stuff like the apocrypha and pseudepigraphica -- stuff written around the time of the bible, some of it referenced by the bible itself... and what makes those not the word of god, when the bible is?