Technically, nothing happens. The drinker's chugged what he should've applied, and claiming to have applied the magic oil on the inside doesn't count. (Unless maybe the drinker's a gully dwarf; those dudes can have weaponized innards.) Drinking oil of bless weapon wastes oil of bless weapon just as (in most cases) using oil of bless weapon as a lubricant or salad dressing would. It's a sad fact that oils are sometimes consumed (and, although rarer, potions sometimes applied) when adventurers can't properly identify a substance. However, a generous DM can rule that not all is lost if the hapless fighter accidentally (or purposefully) consumes a potion of bless weapon…
The drinker could have holy spit
The Wizards of the Coast Forgotten Realms Web column "Halflings in Chessenta" on Ursuma Pepper Powder says
This powder is ground from very hot peppers imported from Chondath and mixed with a secret alchemical substance. When put in a person's mouth, the user takes 1 point of subdual damage. One round later, the pepper powder can be spat into a foe's eyes. If the spitter hits with a ranged touch attack (10-foot range increment), the recipient must succeed at a Fortitude save (DC 13) or take 1d6 points of subdual damage and be blinded for 1d6 rounds. "Spitting" is considered an exotic weapon. Unless proficient, spitters suffer the standard -4 penalty to spit attacks with Ursuma pepper powder. Ursuma pepper powder is not sold, and therefore has no price. Its weight is negligible.
Emphasis mine. Despite the above item's absurdity, squeeze that description hard enough and some rules can be inferred, or, at least, some house rules can be made using that description as a guide. That is, presumably, even without any ursuma pepper powder, any creature with a mouth can spit in another creature's eyes by making a successful ranged touch attack that suffers a −4 nonproficiency penalty if the spitter lacks the feat Exotic Weapon Proficiency (spitting) or the equivalent. But, like the damage dealt by a grig monk's shuriken, the DM will likely rule—as I do—that the typical creature's typical spitting deals no damage. (I also clarify that spitting does not even do 0 points of damage plus the spitter's Strength modifier; your six pack and big guns don't help here.) In other words, a typical creature's typical spitting doesn't do anything, but, even if it doesn't, sometimes it's reassuring to know you still have a ranged weapon when your hands are full. That, and spitting on someone as an attack can be deeply satisfying.
So this DM would allow a creature that consumed instead of applied oil of bless weapon to have for the oil's duration holy spit. The drinker's ability to bypass some creatures' damage reduction with its holy spit wouldn't matter, but if the drinker managed to hit an incorporeal evil creature with its spit, that creature will probably at least be freaked out. (Or as freaked out as an allip or ghost or phane or whatever can be.)
(Note that my house rules set the typical creature's spitting range increment to 5 ft. with a maximum of 5 increments like a thrown weapon, which is, I admit, still too far but remains significantly less than the what-seems-to-be-100-ft. maximum range of an ursuma pepper powder-sucking halfling.)