tach is off oil pressure
Nope. The tacho simply runs off the RPM. It's integrated in the RPM instrument after all.
All tachos are calibrated to a number, typically somewhere in the 2350-2500 RPM range. At that calibrated number, the tacho runs "true", so 1.0 tacho hour equals 1.0 wall clock hours. (The number might be stamped on the instrument, but typically on the side or on the back, where it's not normally visible.)
Rev the engine at anything higher than the calibrated RPM and the tacho runs faster. Anything lower and the tacho runs lower. So if you're doing aerobatics, running the engine at its highest RPM setting all the time, you may easily clock up 1.2 or 1.3 tacho hours in one clock hour. But in contrast, in a slow cruise you may only run up 0.8 tacho hours in one clock hour. And even when idling the tacho runs. Although it will only show around 0.2 or 0.3 per clock hour.
Since the tacho time is much better related to engine wear and fuel/oil consumption compared to hobbs time, tacho time is used in a lot of places to calculate the aircraft rental fee.
Comparing hobbs (regardless of how it's wired) to tacho time may also flag abnormal behaviour by renters. For instance, if each one-hour bimble normally shows 0.8 tacho hours for each 1.0 of hobbs hours, and all of a sudden you find someone who had 1.1 tacho hours for 1.0 hobbs hours, you know you've got to ask some questions. Because they may be abusing the aircraft.