The OACL was one of Lebanon's few multi-sectarian parties, with Christian, Muslim and Druze members, but its main base was among Shi'a Muslims. OACL played a major role in the political radicalization of the Shi'a community during the 1970s. In the 1980s, it had a membership of about 2000.
The Organization of Lebanese Socialists was led by Muhsin Ibrahim and Muhammed Kishli. It had its roots in the Lebanese branch of the Arab Nationalist Movement (ANM), a radical pan-Arab movement. During the 1960s Ibrahim was a leading figure in the leftist tendency with the ANM. This tendency, led by Naif Hawatmeh, argued that the ANM ought to adopt a Marxist outlook. This was opposed by the top ANM leader George Habash who, although being open to introducing Marxist concepts like imperialism into the discourse of the ANM, wanted to retain the anti-Communist character of the organization.
As the central leadership of ANM had shifted to Damascus, the Lebanese branch began to function more autonomously. The official ANM organ al-Hurriya ('Freedom'), of which Ibrahim had become editor in 1960, became a de facto mouthpiece for the Marxist sector. In 1968 the Lebanese branch of ANM broke its links to the mother organization, and renamed itself as the Organization of Lebanese Socialists.
Socialist Lebanon was a small group of Marxist intellectuals, Arab nationalists and former Baathists which had been formed in 1965. Leading members included people like Ahmed Beydoun, Waddah Sharara, Fawwaz Trabulsi and Ismat Kawwas.
OACL criticized the Lebanese Communist Party led by George Hawi for 'reformist tendencies', but held unsuccessful talks on a party merger in the mid-1970s. OACL had a profound impact in the radical student movement that emerged in Lebanon in the early 1970s.
In 1973 OACL took part in the formation of the Lebanese National Movement (LNM) together with the Progressive Socialist Party, the Communist Party, Nasserists and others. Ibrahim became the General Secretary of the LNM.
In 1973-1974 OACL was active in the Ghandour food factory strike.
The OACL then played a leading role in the early phases of resistance to the Israeli occupation. According to Hawi, the Lebanese Communist Party, the OACL and the Arab Socialist Action Party led by Hussein Hamdan had agreed to establish a specifically Lebanese resistance formation before the expulsion of the PLO. Hawi claims that the joint communique calling for resistance to the Israeli occupation of Beirut was written by himself and Ibrahim on 15 September 1982, while the initial resistance activities against the Israelis were undertaken jointly by the Communist Party and the OACL along with some smaller organizations, with Hawi and Ibrahim meeting daily in secret to coordinate activities.
During the 1980s the Syrian occupation of Lebanon grew stronger, and the OACL's alliance with Syria weakened; by 1987 the OACL was forced underground, since Muhsen Ibrahim refused to go along with the Syrian policy of opposition to PLO head Yasir Arafat.