The nature of Oswald the Pardoner
Though it's never stated outright one way or the other, several aspects of Oswald's design and dialogue imply he might be not human, and instead belong to the group of people directly associated with the gods and populating Anor Londo.
Depending on interpretation, the description on Velka's Rapier outright states as much. "Inhuman" can easily be read as figurative language here, simply conveying that he's extremely good; but in the context of other evidence, it could also make sense to take it as a literal description, too: the pardoner is not human.
One such suggestion is the result of attempting to drain Oswald with the Dark Hand. Unlike a majority of other human-looking NPCs, he doesn't provide any humanity when the Dark Hand is used on him. Other NPCs who give zero humanity are largely those near to going hollow (the Crestfallen Warrior & Merchant, Oscar of Astora), those with lore-congruent personal explanations for not having or bothering to have any humanity to spare (Knight Kirk, Big Hat Logan), or those who are confirmed or easily surmised to not be human (Quelana of Izalith, Lord's Blade Ciaran). Oswald fits neither of the first two categories - per dialogue, he explicitly is "pleased to see [the PC] preserving thine humanity".
Other aspects of Oswald's dialogue also hint in this direction. In general, he speaks with similar archaicized grammar as other extremely old characters associated with Anor Londo - Gwyn's children, the Giant NPCs, Artorias (in cut dialogue), Dusk and Elizabeth of Oolacile, Alvina. And there's also his extremely sardonic delivery of his parting line to the PC upon exiting a conversation, with its exaggerated emphasis on "only human". Again, easy to read as the figurative use of the phrase, but considering the delivery and Fromsoft's use of double meanings elsewhere, it's worth considering that it might also serve as a commentary on humans coming from someone who isn't.
- Velka's Rapier description: "It is no mere symbol to be sure; the pardoner is an inhuman swordsman, and wields this enchanted blade with special sword technique"
- Dark Hand humanity drain - none available
- Archaic grammar & manner of speaking
- Conversation exit line: "It is only human to commit a sin!"
Bulleted list on the right of evidence sources. (In this case, his speech (ye olde & ~~oonly huuuuuman to commit a sin~~!), his Dark Hand result (nothing), and the rapier description "inhuman swordsman".)
Little square picture to illustrate, also on the right? Prior to bullet list