The story
A presumably 'natural' exclamation, attested in writing since 1921. Related to or a variation of whoops (itself attested since 1933). A shortening of whoops-a-daisy, whoopsie-daisy, or oops-a-daisy, which in turn is a mispronunciation of ups-a-daisy or upsy-daisy.
Etymology from Wiktionary, CC BY-SA 4.0.