Oscar Greeley Clendenning Hammerstein II (; July 12, 1895 – August 23, 1960) was an American lyricist, librettist, theatrical producer, and director in musical theater for nearly 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.

He is best known for his collaborations with composer Richard Rodgers, as the duo Rodgers and Hammerstein, whose musicals include Oklahoma!, Carousel, South Pacific, The King and I, Flower Drum Song, and The Sound of Music. Described by Stephen Sondheim as an "experimental playwright", Hammerstein helped bring the American musical to new maturity by popularizing musicals that focused on stories and character rather than the lighthearted entertainment that the musical had been known for beforehand.

He also collaborated with Jerome Kern (with whom he wrote the 1927 musical Show Boat), Vincent Youmans, Rudolf Friml, Richard A. Whiting, and Sigmund Romberg.