Adam Andrzej Ostrowski (Polish pronunciation: [ˈadam ɔsˈtrɔfskʲi], born 15 May 1980 in Łódź, Poland), better known as O.S.T.R., is a Polish rapper, musician, audio engineer and record producer, famous for his freestyle rap skills, ambitious lyrics and unique beats.

He graduated from the Academy of Music in Łódź, violin class.

He has collaborated with American rappers such as Evidence, El Da Sensei, Craig G, Keith Murray, Jeru The Damaja, Lil' Dap, Sadat X and Canadian producer Marco Polo.

He has also collaborated with many Polish artists, including jazz musicians Michał Urbaniak, AbradAb Hades, SOFA, Fokus, DJ Deszczu Strugi, DonGURALesko, Vienio i Pelson, Fu, Pezet, Sistars and Slums Attack.

In 2013, he shared the position of art director of Męskie Granie festival with alternative rock vocalist Kasia Nosowska.