Which of these gauges provides direct indication of the oil system operation?

The oil pressure gauge is designed specifically to monitor the pressure of the oil circulating in the engine. This gauge reflects the performance and condition of the oil system, which is critical for maintaining engine lubrication and overall health. A proper oil pressure reading indicates that the oil is adequately circulating through the engine to reduce friction and prevent overheating. If the pressure is too low, it could signal a potential issue such as low oil levels or a malfunctioning oil pump, which can lead to engine damage. Therefore, the oil pressure gauge provides a direct and vital indication of the oil system's operation, which is essential for the safe functioning of the aircraft's engine.

Other gauges, such as the fuel gauge, altitude gauge, and speed indicator, do not provide information about the oil system and are instead focused on other aspects of flight performance and navigation.