Oskar Kreuzer first featured at the German International Championships in 1906, when he won the mixed doubles title, partnered by Nelly Schmöller, and made the final of the men’s doubles. Kreuzer was German Champion in singles in 1920 and in doubles in 1920 (with Ludwig, Count von Salm-Hoogstraeten) and 1922 and 1925, both with Otto Froitzheim. Kreuzer played Wimbledon in 1907-08, 1913, and 1927, also entering in 1910. In 1908 he won the All-England Plate, the consolation tournaments for those who lost in the first and second rounds. At the 1912 Stockholm Olympics he won a bronze medal and, partnered by Otto Froitzheim, the World Championship on hard courts in 1912 and 1913. He played three Davis Cup ties in 1913-14 for Germany, winning one of five matches.
Kreuzer also played top level rugby, winning the 1910 German title with SC 1880 Frankfurt. In 1914 Kreuzer was playing a Davis Cup match for Germany in Pittsburgh (USA) when World War I erupted. On his return, along with Otto Froitzheim, their ship was stopped by the British Navy near Gibraltar, and the two were taken prisoner. They were kept in Gibraltar for several months before being sent to Britain as prisoners-of-war, where they remained until the war’s end. In marked contrast to his countryman, however, who had the privilege of being interned at Donnington Hall in Herefordshire, Kreuzer had to bide his time at a POW camp near Leeds.