A heavy petroleum oil feed is upgraded by having its amenability to cracking improved by subjecting the oil to selective partial oxidation with a catalytic oxidation system to partially oxidize aromatic ring systems in the heavy oil. The partially oxidized oil can then be cracked in the conventional manner but at lower severities to lower molecular weight cracking products. The cracking following the partial oxidation step may be thermal in nature as by thermal cracking, delayed, contact or fluid coking or fluid catalytic cracking or hydrogenative as in hydrocracking.