Measurements of drag reduction in two-phase oil-gas slug flow are presented in this study. Two types of oil with markedly different viscosities were examined in 10-cm inner diameter horizontal pipes to evaluate the influence of oil viscosity on the total pressure loss and the effectiveness of drag-reducing agents (DRAs) in reducing the pressure drop in slug flow The total pressure drop in the 50-cP oil was always more significant than in the 2.5-cP oil, especially when increasing the gas flow rate. However the DRA was more effective in reducing the total pressure drop in the 2.5-cP oil. Furthermore, increasing liquid velocity thus increasing liquid volume fraction, resulted in all increase in the DRA effectiveness for both oils.