Not much is known about Oliver's close friends from the books. In the fan fictionfanfiction world, he is a widely popular character who is close with all his Gryffindor House team mates, especially Katie Bell. There is a large amount of fan fiction which pairs these two, though there is nothing in the series to support this, and in fact it seems unlikely with Oliver being three years older than Katie. He is also often paired with Percy Weasley, who is in his year and he shares a dorm with.
