One idea could be that the OC is a new student at Hogwarts who is sorted into a different house than Draco. They initially clash because of their different house loyalties, but then gradually find common ground through their shared love for magical creatures. For example, they might both be interested in thestrals. Another plot could involve the OC being a muggle - born with extraordinary magical abilities that Draco is both intrigued and threatened by. He tries to expose her at first but then ends up protecting her from other Slytherins who have more malicious intentions.
Well, a plot could revolve around the OC being a transfer student from a foreign wizarding school. Draco is assigned to show her around Hogwarts. At first, he's his usual snobbish self, but the OC doesn't take his attitude lying down and challenges him at every turn. This leads to a series of humorous and tense encounters. As time passes, they discover they have a similar sense of adventure when they accidentally stumble upon a hidden part of the castle together. They have to work together to keep it a secret from the other students and faculty, which brings them closer.