Connect the (-) terminal of the hourmeter to ground and connect the (+) terminal to one of the connectors on the pressure switch. The other connection on the switch gets wired to either the (+) side of the coil or a constant hot 12V source. As soon as the oil pressure comes up, the hourmeter will run. Try an AC-Delco # D1834 switch. It's the 3-prong type that I use regularly. Use a multimeter on the ohms setting to find the two prongs that have continuity with pressure on the switch, and ignore the grounded one.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!