Doh! is now a word according to Oxford

Doh! this is no joke, the following is quoted from the Oxford English Dictionary

doh, int. colloq. "Expressing frustration at the realization that things have turned out badly or not as planned, or that one has just said or done something foolish. Also (usu. mildly derog.): implying that another person has said or done something foolish (cf. DUH int.)."
"1952 A. BUCKERIDGE Jennings & Darbishire ixii. 183 Doh! An anguished gasp of exasperation rang out loud and clear as Mr Wilkins found his voice again. 1989 Beano 11 Feb. 23 (caption) [Speaker is a man who is knocked against a bus stop.] Doh! 1991 Chicago Tribune (Nexis) 15 Nov. (Friday) H, The movie had one good point: It wasn't the worst movie I've ever seen. It was the worst movie I've ever seen. Doh! 1993 HP Professional (Nexis) July 28 Along their long path ISO sort of missed local area networks and network management, which gave the market over to TCP/IP and related technologies. As Homer Simpson would say: Doh! 1996 A. FEIN et al. Simpsons Comics strike Back! 14/2 Look out, you dern fool! You're gonna cut off your… D'oh!!! 1998 N. JONES Hollyoaks (Mersey TV transmission script) Episode 256 44 Cindy: What are we doing here, anyway? Paul: Doh! Use your head, eh?"

reference: Oxford English Dictionary