Premature self-ignitions in hydrogen internal combustion engines have been associated with the presence of hot spots. However, local increases in charge reactivity may be triggered not only by elevated temperatures but also by composition inhomogeneities. Such non-uniformities, in addition to imperfect mixing (e.g., in the case of direct hydrogen injection), may result from external contamination by more reactive components, such as lubricant oil. The present study aims to shed light on the mechanism through which lubricant oil contamination leads to the formation of sensitive spots, by analysing the behaviour of an isolated droplet suspended in a hydrogen/air environment. The “HyLube” chemical kinetic mechanism was employed to reproduce the chemical behaviour of lubricant oil, as it was specifically developed for this purpose. A one-dimensional numerical model was used to simulate the heating, vaporization, and combustion of the droplet. Zero-dimensional simulations were also performed using the open-source code Cantera (vers. 3.0.1) to better asses the effects of contamination. The impact of key operating conditions – such as ambient temperature, pressure, and composition, as well as droplet temperature and size – was investigated. The temporal evolution of selected variables, including the calculated instantaneous local charge reactivity, was analysed to characterise the fundamental mechanisms through which lubricant oil alters the charge reactivity and promotes early flame development within the combustion chamber of hydrogen-fuelled engines. The results contribute to a better understanding of how pre-ignition may occur due to lubricant oil contamination.