This paper details the exploration of oil jet piston cooling phenomenon with a focus on heat transfer from the diesel engine piston to the oil. Several numerical methods based on computational fluid dynamics (CFD) and conjugate heat transfer (CHT) were developed to resolve key aspects of piston oil cooling. These methods aim to establish and characterize the flow and heat transfer regimes that are inherent to the piston gallery cooling system, and to assist in quantifying the piston heat transfer and establish its dependence on a number of parameters related to the engine layout and performance, the oil cooling system, and the cooling gallery contained within the piston. Telemetry experimental data from a single-cylinder diesel engine was used to better understand the piston cooling system and to develop and validate modeling and simulation approaches. The combined findings offer a foundation for further study of oil jet piston cooling.