Whack-O! is a British sitcom TV series starring Jimmy Edwards. It was written by Frank Muir and Denis Norden.

The series (in black and white) ran on the BBC from 1956 to 1960 and (in colour) from 1971 to 1972. Edwards took the part of Professor James Edwards, M.A., the drunken, gambling, devious, cane-swishing headmaster who tyrannised staff and children at Chiselbury public school (described in the opening titles as "for the sons of Gentlefolk"). The Edwards character bore more than a passing resemblance to Sergeant Bilko as he tried to swindle the children out of their pocket money to finance his many schemes.

The first six episodes were subtitled "Six of the Best". In 1959 a film was made based on the show, called Bottoms Up!. The series was revived in colour with updated scripts in 1971–72, retitled Whacko!. In all, it ran for a total of 60 episodes, with 47 of black-and-white and 13 colour, of 30 minutes each. There were three special shorts. There was also a radio version with Vera Lynn starring as herself in the second episode. Many of these radio episodes were recovered by a BBC archivist from a listener's collection of tapes in 2012.

The front of the historic house of Great Fosters near Egham in Surrey was used in the opening title sequence of the TV comedy series, behind the name of the fictional Chiselbury School.