Lubricating oil systems play a critical role in ensuring the efficient operation, long-term reliability, and overall availability of turbomachinery trains (or strings). Any shortcomings in their design can lead to suboptimal performance and compromise not only the oil system itself but also the reliability and functionality of the entire equipment train it supports. To ensure optimal performance, the scope and configuration of the lubricating oil system for a given application should be collaboratively assessed by the responsible equipment vendor, the oil system designer, and the specifying machinery engineer. A thorough review of all system components is essential, and the design basis should be agreed upon by all parties as early as possible—ideally before the purchase order is issued. This chapter explores the key considerations for selecting and sizing the components that make up a pressurized lubricating oil system for modern turbomachinery strings. The design principles outlined are broadly applicable to oil systems serving a variety of driven and driving equipment types.