This tutorial discusses the selection and sizing considerations for components and elements that constitute the pressurized lubricating oil system for a modern turbomachinery string. The design principles explained in this text are commonly applied to oil systems that serve different types of driven and driving equipment. Hydraulic oil systems for gas turbines are not covered in this text. Auxiliary components and accessories, such as oil accumulators, overhead rundown tanks (atmospheric and pressurized), oil mist eliminators and oil conditioners are elaborated in the following sections. It also presents a sample Hazard and Operability Study (HAZOP) of API 614 oil system with typical causes, consequences, severity, and barriers.