Oscar Greeley Clendenning Ritter von Hammerstein II (/ˈhæmərstaɪn/; July 12, 1895 – August 23, 1960) was an American librettist, theatrical producer, and (usually uncredited) theatre director of musicals for almost 40 years. He won eight Tony Awards and two Academy Awards for Best Original Song. Many of his songs are standard repertoire for vocalists and jazz musicians. He co-wrote 850 songs.

Hammerstein was the lyricist and playwright in his partnerships; his collaborators wrote the music. Hammerstein collaborated with numerous composers, such as Jerome Kern, with whom he wrote Show Boat, Vincent Youmans, Rudolf Friml, Richard A. Whiting, and Sigmund Romberg, but he is best known for his collaborations with Richard Rodgers, as the duo Rodgers and Hammerstein, whose collaborations include Oklahoma!, Carousel, South Pacific, The King and I, and The Sound of Music.