Optimisation and Analysis Toolbox for power Systems analysis (OATS) is an open-source simulation tool for steady-state analyses of power systems problems. It contains implementations of classical steady-state problems e.g., load flow, optimal power flow (OPF) and unit commitment, as well as enhancements to these classical models relative to the features available in existing widely used open-source tools. Enhancements implemented in the current release of OATS include: a model of voltage regulating on-load tap-changing transformer, load shedding in OPF, contingency selection in security constrained OPF and the ability to model zonal transfer limits in unit commitment. The mathematical optimisation models are written in an open-source algebraic modelling language, which offers high-level symbolic syntax for describing optimisation problems. The flexibility offered by OATS makes it an ideal tool for teaching and academic research.