Damascus had asked the watchdog to investigate what it called two “mortar attacks with poisonous gas” in July and August 2017 during a government offensive against the Free Syrian Army and Islamist rebels.
Syria said three soldiers were taken to hospital with symptoms including breathing difficulties, muscle spasms and frothing at the mouth, and alleged that the attack involved chlorine gas.
It said four casualties in the August incident suffered similar symptoms.
OPCW investigators made several trips to Syria during which they interviewed 18 people including casualties, and gathered evidence including photos and videos from the hospital, plus medical records.
But the watchdog said it “faced challenges in collecting sufficient information”.