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(Raden Mas) Hadji Oemar Said Tjokroaminoto (August 16, 1882 – December 17, 1934) was a nationalist, the first leader of Sarekat Dagang Islam (Islamic Trade Union, later Sarekat Islam) in Indonesia.
While attending NIAS (Nederlands-Indische Artsen School/ Netherlands Indies Medical College) in Surabaya, Kartosuwirjo boarded at the house of Islamist leader Tjokroaminoto and became actively involved in Tjokrominoto's PSII (Partai Sarekat Islam Indonesia/ Indonesian Islamic Union Party).