Chang 8 layer in northeastern Zhiluo oilfield of Ordos basin is a structural lithologic reservoir controlled chiefly by lithology with many oil water layers. There are many oil water layers in this region. Aiming at this characteristic, on the basis of the methods of pressure coring, mercury injection and log data interpretation, the data of relative permeability analysis is used to to interpret and revise the original oil saturation in the area. The four methods are used to calculate the original oil saturation in Chang 8 layer of northeastern Zhiluo oilfield and the advantages and disadvantages of each method are analysized. The reasonable original oil saturation in Chang 8 layer of this area is determined, and the accuracy of calculation of reserves is improved. The method of relative permeability analysis data is the best method to calculate the original oil saturation in this area, eventually the original oil saturation value is 45.6%.