The function of an operating system (OS) in a computer is to act as an intermediary between hardware and the user applications. It manages the system's resources, including the CPU, memory, and storage, ensuring that different programs run smoothly without conflicts. The OS provides a user interface, which may be command-line or graphical, and facilitates file management and security permissions. Common operating systems include Windows, macOS, and Linux. Overall, the OS enables users and applications to interact with the computer hardware effectively.