Oscar Greeley Clendenning Hammerstein II (pronounced /ˈhæmərstaɪn/; July 12, 1895 – August 23, 1960) was an American writer, theatrical producer, and (usually uncredited) theatre director of musicals for almost forty years. Hammerstein won eight Tony Awards and was twice awarded an Academy Award for "Best Original Song", and much of his work is part of the unofficial Great American Songbook. He wrote 850 songs. Hammerstein was the lyricist and playwright in his partnerships; his collaborators wrote the music. Hammerstein collaborated with composers, including Jerome Kern, Vincent Youmans, Rudolf Friml and Sigmund Romberg, but his most famous collaboration was with Richard Rodgers.

The pronunciation of Hammerstein's surname is often anglicized, to /ˈhæmərstiːn/ HAM-ər-steen. Hammerstein himself, however, kept a more German pronunciation of the vowels, /ˈhæmərstaɪn/ HAM-ər-stine.

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