We have investigated the catalytic treatment of a crude algal liquefaction bio-oil in supercritical water to discover how the properties of the treated oil depend on the experimental conditions. An L₉ (3⁴) orthogonal array design (OAD) with four factors at three levels was employed. The four factors were temperature (varied from 430–530 °C), time (varied from 2–6 h), catalyst type (Pt/C, Mo₂C, HZSM-5), and catalyst loading (varied from 5–20 wt%). We used a direct analysis to determine the relationship between experimental conditions and properties of treated oils. The oil properties we examined were elemental composition, atomic ratios, chemical composition, and higher heating value. Of the four factors, the 100 °C variation in temperature was always the most influential for each of the oil properties examined. Of the remaining three factors, catalyst type had the greatest influence on the fatty acid content of the treated oil and the fraction of N- and O-containing compounds in the oil. Catalyst loading had the greatest effect on the higher heating value and O/C ratio in the treated oil. Reaction time had the greatest effect on the H/C and N/C ratios. The results demonstrated that treatment in supercritical water at 430 °C led to roughly a halving of the N and O content of the oil, a reduction in S to below detection limits, and about a 10% improvement in the higher heating value of the bio-oil. Within the parameter space investigated, the conditions leading to the highest content of saturated compounds in the treated oil are 430 °C, 6 h, with a 10 wt% loading of Mo₂C as the catalyst. Around 76 wt% of the carbon in the feedstock was retained in the treated oil at these conditions.