Simulations of cosmic-ray (CR) trajectories within the Earth’s magnetosphere are crucial for investigating how said CRs interact with our planet. Conducting these simulations is no easy task, as CR trajectories can be heavily influenced by the Earth’s magnetic environment. As a result, these simulations require sophisticated programs that can perform computationally intense numerical integration to resolve these trajectories. There are multiple tools that have been developed to do this very task, however, these tools can be challenging to work with by being hard to access, no longer supported, and difficult to edit. In order to address the community’s desire for a new tool to conduct such computations an alternate new open-source program named the “Oulu – Open-source geomagneToSpheric prOpagation tool” (OTSO) has been developed. This tool aims to supply the community with a user-friendly tool that can conduct the necessary computations required for CR study in the magnetosphere, whilst also providing a robust foundation a tool that can be developed further by the community to meet the field’s needs, this removes the need to constantly develop new programs once the old ones become abandoned. OTSO has been able to replicate the results of an older widely used and validated tool, MAGNETOCOSMICS, and was successfully used to analyse two ground-level enhancement events, obtaining results in good agreement with prior studies and in-situ space-borne measurements.