The precursor to NK Osijek was founded on 27 February 1947, after a merger between two physical training associations: Slavonija and Bratstvo. The new entity was named Proleter. The first match that was played under that name dates back to 16 March that same year, when Proleter beat city-rivals Mladost by five goals to nil. The first competition in which the club participated in was the Osječko Okružno Prvenstvo, along with four other teams. The club got into the second national league soon after. Proleter achieved placement into the Yugoslav First League in 1953, having won the so-called Croato-Slovenian League. The best players from that side were Andrija Vekić, Franjo Rupnik, Dionizije Dvornić and Franjo Majer. Proleter played in the First League for three seasons, but were then relegated to the second division.