The story
Imitative. Popularised by the cartoon character Homer Simpson in the TV show The Simpsons. Voice actor Dan Castellaneta has said that he modelled his version on the drawn-out "do-o-o-o" sound made by Jimmy Finlayson in the films of Laurel and Hardy. The OED has references from the BBC as far back as 1945.
Etymology from Wiktionary, CC BY-SA 4.0.