Ottokar Gräbner was born in Osterwick near Konitz, which at the time was in Western Prussia. In 1923/24 he began an apprenticeship with the wood carver Seipold (Seibold) in Danzig and took drawing lessons from Bruno Paetsch. In 1925 he became a student of Ludwig Meidner in Berlin. From 1926 to 1929, Gräbner worked as a freelance painter in Danzig and exhibited his first paintings in group exhibitions. From 1930 to 1933 he was a master student of Max Beckmann at the Städel Institute and worked in the graphic class of Franz Karl Delavilla.
He was active again in Danzig as of 1933, but was banned from exercising his profession by the Nazis, and was persecuted by them. Gräbner emigrated to Poland in 1936, and after the outbreak of war in 1939 he moved to Soviet-occupied Poland. In 1943 he returned to Germany.