He is considered as the most notable football player from Opatija,[1] and one of the five greatest forwards in the early history of HNK Rijeka (along with Bruno Veselica, Miodrag Kustudić, Milan Radović and Tonči Gulin).
In 1953 he was signed by Dinamo Zagreb and this time he appeared in 22 league matches and scored 11 goals over the next three Yugoslav First League seasons, appearing alongside other Dinamo greats such as Željko Čajkovski, Tomislav Crnković, Dionizije Dvornić, Vladimir Čonč and Luka Lipošinović.
He is mainly remembered for his performances in the title-winning 1953–54 season, in which he scored 9 goals in 10 league appearances for the Blues, including the crucial goal against Red Star Belgrade at Maksimir on 25 April 1954, which sealed Dinamo's second championship triumph.
After retiring from active football, Osojnak started working as manager and had three managing spells at NK Rijeka (1960–1961, 1962–1963 with Angelo Zikovich and 1964–1967), his greatest achievements being finishing 4th in the Yugoslav championship in 1964–65 and 1965–66.
He was reputed to be the oldest living Dinamo player at the time of his death and was 'succeeded' by then 91-year old Željko Čajkovski.