Oscar Ryszard Lange (born July 27, 1904 in Tomaszów Mazowiecki, Poland - died October 2, 1965 in London, United Kingdom) was a Polish economist and diplomat. He is probably most known for his work On the Economic Theory of Socialism published in 1936. He was an advocate of using market tools (especially the neoclassical pricing theory) in economic planning of socialism and communism. He also worked on integrating classical and neoclassical economics into a single theoretical structure. In terms of economic philosophy, he is sometimes considered to be a "market socialist".
Lange emigrated to the United States in 1937, was a professor at the University of Chicago from 1938, and became a U.S. citizen in 1943. Lange served as a go-between for Franklin Delano Roosevelt and Joseph Stalin during the post-war discussion on Poland. Lange returned to the United States at the end of May, met with Free Polish London Exile Prime Minister Stanisław Mikołajczyk, who happened then to be in Washington, and stressed how reasonable Stalin was prepared to be, and asked the State Department to pressure on the exiled Poles. Lange's covername with the KGB was FRIEND.
Stalin had personally asked Presidential adviser Harry Hopkins for Lange to participate in the Communist dominated Lublin regime.  Lange renounced his American citizenship and returned to Poland. In 1946 Lange arrived again in Washington as the Lublin Committee's first Ambassador to the United States from 1946 to 1947.
A university in Poland is named after him, the Akademia Ekonomiczna we Wroclawiu im Oskara Lange (the Wrocław University of Economics of Oskar Lange).
- Roosevelt and Hopkins : An Intimate History, Robert E. Sherwood, New York Harper and Brothers, 1948, pgs. 934, 941 pdf.
- Charles Sadler, Pro-Soviet Polish-Americans: Oskar Lange and Russia's Friends in the Polonia, 1941–1945, Polish Review, 22., no. 4 (1977), 25–39.
- Venona 956-957 KGB New York to Moscow 6 July 1944 states that KGB agent Boleslaw Gebert had reported on 30 June 1944, "the other day Lange presented to the State Department and to Roosevelt’s Secretariat a report on the journey to the USSR and the conversation with Comrade Stalin. Gebert read this report." 
- FBI Silvermaster file Vol. 83, pgs. 83 -84 pdf, Hottel to the Director, January 8, 1947. Nathan Gregory Silvermaster with “the Minister” and Oscar Lange. Poles are negotiating for a loan and Silvermaster is conferring with them for disposition of surplus materials to be purchased through private firms.