Experiments on flow patterns during air-oil two-phase flow in a long pipeline-riser system with an S-shaped riser are carried out. The test loop with 50.8 mm internal diameter consists of a horizontal pipeline with 119 m in length, followed by a 14 m downward inclined section, and ended at an S-shaped flexible riser with 24 m in length, 15.3 m in height. The inclination angle of the downward section is -2° from the horizontal pipeline. Based on the analysis of the pressure at the riser base and liquid holdup at the riser top, the flow regimes observed are classified into three categories: severe slugging of type 2 (SS2), transition flow (TRS), stable flow (including bubble flow, slug flow and annular flow). In contrast with air-water two-phase flow, there is no sign of severe slugging of type 1 in the present study. As for SS2, the probability density function (PDF) of liquid holdup presents a bimodal distribution with two peaks at HL = 0 and HL = 1, respectively. As for TRS, the PDF curve produced by liquid holdup data presents a unimodal distribution with a single narrow peak at HL = 0. As for stable flow, the PDF of liquid holdup presents a bimodal distribution between HL = 0.3 and HL = 0.8.