A theoretical and experimental study on unidirectional spreading of oil under solid ice in calm waters is presented. The emphasis of the study is on the dominant spreading mechanism for oil under ice, which is the buoyancy-viscous phase. The equations derived cover both the constant discharge and the constant volume modes. Laboratory experiments were conducted for ice cover roughnesses varying from smooth to rough, using different discharge rates, and oils of different viscosities. The experimental results agree closely with the theory.Key words: oil spreading, oil slicks, oil under ice, buoyancy-viscous phase, constant discharge, constant volume.