Due to the energy transition, an increasing number of oil pipelines will be decommissioned in the foreseeable future. As a part of the decommissioning process, the oil needs to be flushed from the pipelines. Some models to predict the required water velocity to flush oil from pipelines are available, but these are limited in accuracy, and data to validate these models are lacking. The aim of this MSc research project is to gain a better understanding of the oil flushing process in pipelines using water, and to provide validation data for the existing models. Thereto lab experiments were carried out in the flow loop of TNO in Rijswijk. The length of the pipe is almost 6 m and the diameter is 56 mm. The experiments are performed by filling the experimental pipe section with oil and subsequently flushing it using four pipe volumes of water. The first set of experiments, for a pipe inclination of 0° to - 5°, was performed with superficial flushing velocities ranging from 0.05 m/s to 1.5 m/s, for two different types of oil. Data were collected using a mass flow meter system and a video capturing system. These data were subsequently analysed using the software tool MATLAB. The first set of experiments was also modelled in the multi-phase flow modelling tool OLGA. A second set of experiments was performed to find the transition point between stratified and mixed flow, which is relevant for the flushing efficiency, by varying the inclination in a range of 0° to -90° relative to the horizontal plane, using a superficial water velocity of 0.1 m/s. In addition to the mass flow measurements, data were collected with a video system during all sets of experiments, and subsequently processed to determine the phase distribution throughout the pipe during, and after the flushing process. It was established that the flushing efficiency is proportional to the superficial water velocity. For both types of oil, for inclinations between 0° and -5°, a superficial water velocity of 0.35 m/s suffices to flush all oil when flushing four pipe volumes of water. For all considered inclinations and pipe geometries and for both oil types, the bulk (≥ 95%) of the oil is removed from the pipe for a flushing velocity of 0.25 m/s. In a horizontal orientation, the lighter and less viscous Fuchs Renolin DTA7 oil is flushed more efficiently from the pipe compared to the Mobil Velocite No.10 oil. This effect diminishes for downward inclined orientations and is reversed for an inclination of -5°. In OLGA it is possible to easily vary for example the geometry of the pipeline and show the effect on the two-phase flow. Discrepancies were found, however, when the flow was evaluated around the 0° inclination angle. Small deviations from this angle of inclination gave significantly different results. For the 0° to 90° range of inclinations, and a superficial velocity of 0.1 m/s, the transition zone between stratified flow and mixed flow was established to be in a range of 7.5° to 20° relative to the horizontal plane. Either decreasing or increasing the inclination angle from this point is beneficial to the flushing process. Apart from the increased understanding of the flushing process summarised above, the data collected on the flushing process under various sets of conditions can be used to validate models for predicting oil flushing processes.