Accurate modelling of cosmic-ray trajectories within the Earth's magnetosphere is important when analysing the impact of cosmic-rays at Earth as well studying their sources, especially cosmic-rays that have a solar origin. In order to compute these trajectories accurately a sophisticated computational tool is required to perform numerical integration over the particle's motion whilst also providing a realistic model of the Earth's magnetosphere. A new tool named "Oulu - Open-source geomagneToSphere prOpagation tool" (OTSO) is presented here to fulfil this role. A comparison between OTSO and an older, out of date, but widely used verified tool, namely MAGNETOCOSMICS, is conducted for various key geomagnetospheric phenomena. A good agreement is found between the two tools supporting the usefulness of OTSO as a more modern alternative tool which can be developed further by the community due to its open-source nature.