Abbreviation for residual oil saturation. Fraction of pore volume occupied by oil at the end of oil displacement that used a specific fluid. This reservoir engineering quantity signifies the ultimate recovery under a given displacement process and represents the endpoint of the relative permeability curves in reservoir simulation. The residual oil saturation quantity is the saturation achieved after an infinite number of pore volumes of the displacing fluid have flowed through a particular portion of reservoir rock. To define residual oil saturation, the displacement method and the type, volume, direction and velocity of the displacing fluid must be known. Residual oil saturation is the ratio of the immobile residual oil volume divided by the effective porosity.