Oscar Hammerstein I (8 May 1847 - 1 August 1919) was a businessman, theater impresario and composer in New York City. His passion for opera led him to open several opera houses, and he rekindled opera's popularity in America. He was the grandfather of lyricist Oscar Hammerstein II.

Oscar Hammerstein I was born in Stettin, Prussia, (now, Szczecin, Poland), to a German-Jewish parents Abraham Hammerstein and his first wife, Berthe. He took up music at an early age. His mother died when he was fifteen years old, and he fled his father, who maltreated him, to seek his fortunes in the United States, arriving in New York City in 1864. He worked sweeping the floor in a cigar factory. Ten years later, he founded the U.S. Tobacco Journal. He also moonlighted as a theater manager in the downtown German theaters.

He was an innovator in the tobacco industry and held patents for 52 inventions, 44 of them related to the cigar-manufacturing process. He became wealthy industrializing cigar manufacturing, and his tobacco fortune provided the money he used to pursue his theater interests.