When Oskar was enlisted in January 1940, his orchestra continued under Rudi Juckeland.
In 1924, Oskar and his brother Ali formed the Oscar Joost Dance Orchestra, with Oskar spelling his name with a 'c' instead of a 'k'. Oscar played first violin, and his brother percussion.
Oskar attended high school in Leipzig and participated in the First World War as a volunteer.
In 1933, Oskar Joost joined the Nazi Party, again spelling his name with a 'k'.
His father, Albert Joost, was a cellist in the military.