(Raden Mas Hadji) Oemar Said Tjokroaminoto (August 16, 1882 – December 17, 1934), better known in Indonesia as H. O. S. Tjokroaminoto, was a nationalist, one of the leader of Syarekat Dagang Islam (Islamic Trade Union) after the founder Haji Samanhudi, later became Sarekat Islam) in Indonesia.