Transportation of very viscous fluids through pipelines is a real challenge. With the depletion of light oils in reservoirs, it becomes economically favourable to harvest heavy crude oils to contribute to meeting the ever growing energy demand. A suitable candidate for the transportation of these very viscous oil is by means of core-annular flow. Core-annular flow is a flow regime of liquid-liquid two-phase flow, where a low viscous fluid in the annulus (water) is used to lubricate a high viscous fluid in the core (oil). The pressure drop is considerably reduced compared to single phase oil flow at the same oil flow rate. In this study the influence of the oil viscosity on oil-water core-annular flow through a horizontal pipe is investigated experimentally. The fixed oil flow rate is set at 0.35 l/s at which the watercut is varied between 9% and 25% and the oil kinematic viscosity is altered by heating up the oil in a range from 3000 cSt at 20 °C to 400 cSt at 50 °C. Pressure drop measurements for stable core-annular flow are recorded with an electronic pressure transducer and are scaled with the calculated pressure drop of single phase oil flow. Results for the scaled pressure drop at room temperature are compared to results by Ingen Housz et al. It is concluded that for increasing oil-water viscosity ratio, the scaled pressure drop decreases. At the highest considered viscosity ratio, the scaled pressure drop is almost independent of the watercut. Two models to predict the pressure drop (by Brauner and by Bannwart) are evaluated and deviations between the models and measurements are discussed. Visualisation by means of high-speed camera is applied, where a mirror is placed on top of the visualisation section to capture the front and top view simultaneously. For decreasing oil viscosity, the oil-water interface shows a more irregular wave pattern with shorter wave lengths.