This study investigates how carbon dioxide (CO2) curing interacts with raw and calcined fly oil shale ash (FOSA) in ordinary Portland cement paste and mortar (OPC), providing information on previously overlooked synergistic effects of CO2 curing and FOSA calcination. Specimens exposed to CO2 curing followed by water curing were compared to those continuously cured in water. The results reveal that samples containing calcined FOSA as a binder increased compressive strength by up to 200 during the curing stages compared to water-cured samples. Microstructural investigations show that calcium carbonate crystals and other hydration products form, which enhance material properties. The substitution of cement with FOSA results in a reduction in porosity regardless of the curing technique used; however, this issue is efficiently managed by CO2 curing. Higher pH values can be measured from raw FOSA samples, and higher calcination temperatures can result in slightly higher pH values, making the phenolphthalein indicator approach less accurate in detecting carbonation. The application of raw and calcined FOSA as a cement substitute gives long-term benefits while also contributing to environmental awareness in the construction sector. © 2024 The Authors