In generator protections, a low oil pressure alarm is used to alert the operator to the engine suffering from low oil presure. The low oil presure can be monitored by either a oil pressure switch, or an oil pressure sender.
An oil pressure switch is an on engine protection device that is used to monitor the oil pressure of the engine. Normally screwed into the engine block, it has two states, either open or closed. Traditionally an oil pressure switch is closed to earth on fault (when oil pressure is low) and open when the oil pressure is above this minimum requirement. Engine manufactueres determine what the level of low oil pressure is and when the engine should be shut down to prevent problems or catastrophic failure. Switches are often provided by the engine manufacture, rather than the generator manufacturer.
In addition, an oil pressure switch is often used to ensure an engine isn't running before trying to crank. Because an oil pressure switch will be in its fault state, closed to earth, when the engine is at rest (because when the engine is not running, the engine has no oil pressure), it is checked prior to cranking to ensure the engine does not appear to be running, as engaging the starter motor while the engine is running can cause the starter to fail.
To check the oil pressure switch you need a multimeter reading resistance. When the engine is at rest, the oil pressure switch should be closed to earth, when the engine runs it should then open. if the switch does not change its state when the engine runs, check if engine has a low oil pressure condition and/or try another switch.