"Oh, What a Beautiful Mornin'" is the opening song from the musical Oklahoma!, which premiered on Broadway in 1943. It was written by composer Richard Rodgers and lyricist/librettist Oscar Hammerstein II. The leading male character in Oklahoma!, Curly McLain, sings the song at the beginning of the first scene of the musical. The refrain runs: "Oh, what a beautiful mornin'! / Oh, what a beautiful day! / I've got a beautiful feelin' / Ev'rythin's goin' my way." Curly's "brimming optimism is perfectly captured by Rodgers' ebullient music and Hammerstein's buoyant pastoral lyrics."

This was the first song of Rodgers and Hammerstein's musical collaboration to be heard by theatre audiences. It has become one of their most famous numbers and "quickly became one of the most popular American songs to emerge from the wartime era, gaining currency away from Broadway first on the radio and recordings, and then later on numerous television variety shows." Brooks Atkinson, reviewing the original production in The New York Times, wrote that the number changed the history of musical theatre: "After a verse like that, sung to a buoyant melody, the banalities of the old musical stage became intolerable."