I'll start you out. For the tach. The case is ground. There is a wire from the neg. side of the coil, and the +12 that comes on with the ignition. I can't recall what color the wires were, but the power goes to a cap over one of the screws, while the "-" from the coil is a tab that fits under another screw. If this makes no sense, I can E-mail you some pictures and a better description.
As for the oil light question. What year of car do you have, and what engine? My 70 big block has an oil pressure cannister near the back of the engine, behind the carb.. I have rally gauges, so this works for me, I'm not sure about an idiot light. Sorry.
I think you have this right though. Get the oil pressure gauge working, and power to the clock and tach. I can help with the power stuff, I'm only unsure about the oil pressure gauge. I'm not sure if they use the same sensor. :dontknow:
Mike