You say d'oh when you suddenly see that you have done something silly — left your keys inside, given the wrong answer, walked into a glass door. It is a joke sound more than a school word, and it is kind: you are laughing at yourself, not attacking anyone. The cartoon The Simpsons made this spelling famous; Homer Simpson says it with a slap to the forehead. The voice actor, Dan Castellaneta, has said he modelled his version on a long “do-o-o-o” that Jimmy Finlayson used in Laurel and Hardy films. Though actually, English speakers were already making this kind of sound much earlier — there are BBC uses from 1945 — so the show did not invent it. It gave everyone a spelling and a face to go with it. Young people and TV fans use it; it feels casual and a bit cartoonish. You would not put it in a formal letter.