After attending classical grammar school, Otto Freytag took an apprenticeship in technical drawing for engineering and architecture. In 1906 he entered the Düsseldorf School of Arts and Crafts, where he initially studied architecture in the class of Peter Behrens, then switched to studying painting. After completing his training, he became a student of Paul Sérusier in Munich. Freytag accompanied Sérusier on a visit to Paris in early 1909, where his teacher introduced him to the artists’ circle at the Café du Dôme.
In 1910, Freytag returned to Germany and went to Berlin, where he was accepted to the studio of Lovis Corinth. During the First World War, Freytag was a medical orderly in Flanders from 1914 onwards and in 1918, he was stationed in France. From circa 1923 onwards, he exhibited regularly at the “Juryfreie Kunstschau” in Berlin (the “jury-free art exhibition”), until it was cancelled by the Nazi regime in 1934. In the same year, he was awarded the Villa Romana Prize. He lived and worked for a year in Florence. After his return, however, his art was declared “degenerate” in Germany, and his paintings were removed from the National Gallery and the Kronprinzenpalais in Berlin. Freytag was nevertheless allowed to teach at the Art Academy in Berlin-Schöneberg from 1937 onwards. His only son, Kaspar, died in the war shortly before the cessation of hostilities in 1945.