This paper deals with an experimental study on flows in a pipe forced to oscillate horizontally about a vertical axis through the center of its inlet section. The study is concerned with the laminar and transition flow regimes and the flow is investigated using streak lines as a visualization technique. According to a similitude analysis, four criteria govern this phenomenon: the flow Reynolds number, the oscillations Reynolds number (or the reduced frequency), the oscillations amplitude and the Euler number. This paper is mainly concerned with the effects of the oscillations and the flow Reynolds numbers. The results show that the pipe oscillations destabilize the laminar flow: the straight streak line observed in the case of a stationary pipe undergoes a bending and then deforms into an S shape; the longitudinal location of these flow events correlates with both Reynolds numbers. The pipe oscillations are also observed to induce an earlier transition from the laminar to the turbulent regimes; again, the flow and oscillations Reynolds numbers determine the flow regime present in the oscillating pipe. A chart based on the results of 130 tests is proposed to define the conditions of these regimes.