An oik is a rude British insult for a person someone thinks is not intelligent or well educated, and who speaks or behaves in a rough way — often a young man. It is class language: the speaker is looking down. It is informal, unkind, and rather old-fashioned. You will hear it in British stories and old comedy more than in careful speech. Nobody is quite sure where it came from (early 20th century); it may copy a clumsy, uncultivated sound. You should not use it about real people.