Direct proof of use
P-800 Oniks/Yakhont use in the 2011 Syrian Civil War is documented through November 2016 reporting on Bastion coastal-defense launchers firing from inside Syria. CSIS Missile Threat states that Oniks missiles were launched on 15 November 2016 from an unknown location in Syria using a Bastion land-based coastal missile launcher and that the missile targeted Syrian rebel groups.
Other missile-defense and defense-reporting sources attribute the firing to Russia. Missile Defense Advocacy Alliance states that Russia fired Yakhont missiles from road-mobile launchers in Syria on 16 November 2016 as part of a coordinated strike against rebel groups, while a Jane's Defence Weekly article republished by Pan.bg says the Russian Ministry of Defence released footage and said P-800 Onyx missiles were fired against rebel land targets in Syria on 15 November.
Sources: CSIS SS-N-26 Profile, MDAA P-800 Oniks Profile, Jane's Bastion-P Syria Report
Timeline
The direct-use record centers on the mid-November 2016 Russian strike package in Syria. Jane's reported that Russia revealed on 15 November that Bastion-P systems were deployed in Syria and that at least two missiles appeared to have been fired from two different launchers in Ministry of Defence footage. CSIS records the same date for Oniks missiles launched from inside Syria.
MDAA dates the Yakhont firing to 16 November 2016 and describes it as the first use of Bastion in a surface-to-surface capacity. Jamestown, citing Russian Defense Minister Sergei Shoigu's report to President Vladimir Putin, also describes Bastion's first real battle use in Syria as ground-target strikes rather than sea-target engagement.
Sources: CSIS SS-N-26 Profile, MDAA P-800 Oniks Profile, Jane's Bastion-P Syria Report, Jamestown Bastion Foreign Policy Tool
Narrative
The documented Syrian use was not an anti-ship engagement. It was a land-attack strike by Bastion road-mobile coastal-defense launchers using the Oniks/Yakhont missile family. The launch context placed the weapon on the Syrian government and allied side of the war because the firing was part of Russia's intervention in support of Damascus and was directed at anti-government armed groups.
The sources separate possession from use. CSIS notes earlier Syrian acquisition and display of Yakhont missiles, but the conflict-use claim here rests on the November 2016 firing from inside Syria. CSIS records uncertainty over whether the missiles were launched by Russian or Syrian forces; MDAA, Jane's, and Jamestown attribute the relevant strike reporting to Russia or Russian official statements.
Sources: CSIS SS-N-26 Profile, MDAA P-800 Oniks Profile, Jane's Bastion-P Syria Report, Jamestown Bastion Foreign Policy Tool