Estonia has two of the world's largest oil shale firing circulating fluidized bed (CFB) units with a designed electrical capacity of 215 MW each. The units are based on double boiler CFB technology provided by Foster Wheeler Energia OY. The units are located at Eesti and Balti power plants (EPP and BPP). The paper presents analyses of data obtained from tests of oil shale and biomass co-combustion in the full-scale CFB boiler located at BPP. The tests were conducted at nominal boiler load: 100% (314 t/h), with a biomass thermal input of 15%. During the experiments ash samples from the furnace chamber (bottom ash), INTREX, super-/reheater (SH, RH), economizer (ECO), and air preheater (APH), and from all four fields of the electrostatic precipitator (ESP) were taken. Samples of fly ash for determining the mass division (total suspended particulates PM10 and PM2.5) were taken after the ESP. The gas analysis was performed at the ESP inlet. Analysis of the chemical composition of ash was carried out. The specific consumption of oil shale per useful heat and gross electricity were found and other techno-economic characteristics determined. It was found that oil shale and biomass co-combustion reduced CO2 emission by 14.6% and ash formation by 16% when compared with conventional oil shale CFB combustion. The SO2 emissions remained in the limits of 20-30 mg/Nm(3). Total suspended particulates PM10 and PM2.5 did not change compared to conventional oil shale CFB firing. The CFB boiler efficiency even increased slightly, when it is known that in case of coal and biomass co-combustion it decreases. Therefore, oil shale and biomass co-combustion can be considered as a viable option and near-term solution for reducing the environmental impact of oil shale-based power production.