In the Buffyverse, what happens if a vamp bites a werewolf? Or vice-versa?
It’s not been textually established. My presumption is that since the vampire is dead it would not be affected by the werewolf’s bite, would not be infected or turned. Were a vampire to bite a werewolf (and I assume you mean then have the wolf feed on him/her) the werewolf could become a vampire and, being dead, would probably be cured of the lycanthropy.
I think if a vampire were to sire a werewolf, you would probably get a vampire lycanthrope- probably able to shapeshift at will, as long as it was night, being a vampire would expand its supernatural powers or something.
If the vampire just fed on the werewolf, you’d have a dead werewolf, assuming wolfie was in human state when the vampire bit it.
Of course, if you wanted to test the theory, you could put Oz and Spike in a room together and let them rassle it out. There could be some kind of oil…
I like the way you think! Mmm, Spoz…
There’s a very… interesting fanfic about just this sort of thing. Although there wasn’t any oil. 
It hasn’t been established, but in that story at least Oz was turned into a vampire partway through his transformation process, cursing him to a life as a half-werewolf vampire. Spike was unaffected by the werewolf virus.