When AKEL was declared illegal by the British colonial government of Cyprus in 1955, PEO and especially its leader Andreas Ziartides was the only legal leftwing political entity and it was expressing the politics of underground AKEL.
PEO had a membership from all ethnic groups in Cyprus. In 1958 the Turkish Cypriot nationalistic organisation TMT launched a terror campaign against Turkish Cypriot members of PEO, killing the journalist Fazil Onder and attempting to kill the head of PEO's Turkish bureau, Ahmet Sadi. Following those events, most Turkish Cypriots left the union, and PEO's members since then have primarily been Greek Cypriots.
The PEO is affiliated with the World Federation of Trade Unions.