I fixed the oil pressure problem. It had to be electrical. That much was pretty much a given, given that I ripped out and re-routed most of the behind-the-panel wiring yesterday. The oil pressure sensor is a 3-wire sensor using ground, +5vdc, and the sensor output. First, I checked for +5vdc, and that was fine. I didn’t have any way to check the sensor output itself, so next, I checked ground. I had continuity of a sort. Bad, but varying. I stumbled and bumped the plane, and the meter (in continuity testing mode) started beeping. Yep, bad ground. Turns out I missed replacing a ground wire, and it was laying on the floor of the passenger compartment. It would make contact with the metal and work for a bit, then not. Once I finally found it, that was an easy fix.
I started the plane, and everything worked. Well, almost everything. The oil pressure sensor was working again, but my battery voltage was low enough to trip the Master Caution warning, and was reading 12.6vdc. Way way way low for a running engine. Then the alternator light started glowing, softly at first, then full brightness. Crap. Long story short, it turned out to be exactly the same problem. Once that ground wire was fixed, then I was 100% in business again.