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Deep water structures can experience vortex-induced vibrations (VIV) when exposed to marine currents. The engineering challenges of VIV for marine risers are of great concern as oil exploration moves to increasingly deeper waters. The VIV phenomena could cause permanent damages in risers. In practice, offshore floating platforms subjected to waves, currents or winds may cause the risers to move periodically in the water, then to generate relatively oscillatory flows between the risers and the water. Such relative flows can play an important part in the vibrations of the risers. For a top-tensioned riser, the swaying or surging of the floating platform may generate relatively horizontal oscillatory flow between the riser and the water. The vibrations of risers in this type of time-varying currents can be intricate. The time-varying current speeds imply that the vortex shedding frequencies keep going up and down. When the vortex shedding frequencies meet one of the risers’ natural frequencies, lock-in or resonance phenomena occur. VIV in oscillatory flow is numerically investigated based on strip theory in the present work. The algorithm PIMPLE in OpenFOAM is employed to compute the flow field by solving transient incompressible Reynold-averaged Navier-Stokes equations while the small displacement Bernoulli–Euler bending beam theory is used to model the riser. The top end of the flexible riser is forced to harmonically oscillate in both directions based on specified patterns. VIV responses in the cross-flow and in-line directions are studied. The results show that VIV in oscillatory flow is quite different from that in steady flow; features, such as intermittent VIV, hysteresis, and amplitude modulation are observed. Moreover, a VIV developing process including “building-up,” “lock-in,” and “dying out” is further analyzed.