Direct proof of use
Human Rights Watch documented a video posted on 19 August 2013 showing a fighter from al-Fastat al-Muslimeen Brigade, a Free Syrian Army component, firing an M79 Osa anti-tank rocket launcher in Abbasid Square, Damascus.
The same HRW report says the fighter claimed to be targeting the office of the president of the civil defense forces in Abbasid Square. That source supports a direct-use claim for opposition and anti-government forces in the 2011 Syrian Civil War, but it does not establish the launcher's supply route, serial identity, or broader inventory in rebel hands.
Sources: He Didn't Have to Die: Indiscriminate Attacks by Opposition Groups in Syria
Timeline
The supported incident is dated to 19 August 2013, when the video cited by HRW was posted. HRW placed the incident within its wider account of armed opposition attacks in Damascus and Damascus Countryside between January 2012 and April 2014.
Because the public source used here is incident-level, the timeline stays limited to the documented firing rather than treating the M79 Osa as a continuously tracked rebel inventory item throughout the war.
Sources: He Didn't Have to Die: Indiscriminate Attacks by Opposition Groups in Syria
Battlefield role
In this documented case, the M79 Osa appeared as a shoulder-fired direct-fire weapon in an urban Damascus setting. The recorded user was an opposition fighter from al-Fastat al-Muslimeen Brigade, and HRW identified the brigade as part of the Free Syrian Army.
The use case differs from later Islamic State-related M79 Osa evidence elsewhere in the catalog. For the 2011 Syrian Civil War row, the direct support is the HRW-documented Damascus firing incident, not a transfer record or a comprehensive accounting of who held the launcher across Syria.
Sources: He Didn't Have to Die: Indiscriminate Attacks by Opposition Groups in Syria