Temp should read Hot with no power to it (key off). It will also read Hot on a V8 if the single-wire sending unit has failed; do you have a V8 or 6?

Take the wire off the oil pressure sending unit and isolate it (away from ground). The gauge should now read zero with the power on. If it still pegs, the gauge unit is wired up wrong or the wire is shorted to ground between the gauge and the sending unit.