Depends.
If it's only because the OC creator wants their character to be paired up with canon, I find it annoying. They can have that pairing in their head and keep it to themselves, which I'm fine with. but it's only when they start going around and making it obvious when it really annoys me. For example, I've had a few OCxCanon relationships in a few roleplays I've been in. The pairings stay in the RP(s), and outside of them, the OCs are single. If that makes sense.
But normally I'm fairly tolerant of them. It's only when they're paired up for no reason whatsoever and made obvious that I don't like them.
Anyway, I find this DA stamp to reflect my opinion:
link
Sorry, I can go, like, forever on this. xD