Well you see, when It's a fanfic it kind of makes sense, as you won't have to create a characterize one or more potential love interests and that makes it easier. But it's an OC and I'd say that it's safe to assume that whatever happens in a fanfiction about them is the canon for them. Generally having an OC, besides a second gen, that has a significant romantic or even non-romantic relationship with a canon character is looked down upon. That's not to say that it can't be done, it's just hard and usually has people who will ignore the fic altogether just because of the presence of a OC x canon pairing.
I'm not going to sugarcoat it, I really hate it, I just generally cringe away from such things. I'm okay with minor interactions or even writing a fic in the normal episode format, as in Twilight and/or other members of the mane six have to do something that relates to that OC in some way because if you're basically writing your own episode, not a canon one of course, then having some sort of OC makes sense seeing as there may not be a canon character that would make sense to use in the way your planning.