You have some form of switch that you turn on to enable the starter, don't you?
When this switch is on, the low oil pressure alarm
A "true" Hobbs install is leave Hobbs powered when ignition is hot, ground wire is connected to an oil pressure switch so that whenever there is oil pressure, the circuit is completed and the Hobbs runs whenever there is oil pressure, assumption of course being that the only time there is oil pressure is when the engine is running
Most of us have some form of "ignition switch" that when this switch is on, the low oil pressure alarm
is sounding, seeing as how I've never met anyone that will just let that alarm go off without it annoying them, it's a safe bet that whenever the ignition is on, there is oil pressure and its OK for the Hobbs to be triggered just by ignition voltage, without an oil pressure switch, cause that alarm is not going to allow you to forget to turn off the ignition, whether it be a switch or a key.