Adam Ostrowski (Polish pronunciation: , born 15 May 1980 in Łódź, Poland), better known as O.S.T.R., is a Polish rapper and record producer, famous for his freestyle rap skills, ambitious lyrics and unique beats. He is one of the few polish rappers that have actually received professional musical education. Together with WSRH Tarnowski and Seba WSRH, he raised over 54 000 złotych for cancer research.

He has collaborated with rappers such as Evidence, El Da Sensei, Craig G, Keith Murray, Jeru The Damaja and Sadat X. He has also collaborated with many Polish artists, including Michał Urbaniak, Tede, 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 with Kasia Nosowska.