[Injuneer]

GM has pretty much standardized the harness wiring for the heated O2 sensors, and it is the harness wiring you want to splice into, not the sensor wires themselves. The black wires are the heater +12V and ground, the white or tan wire is the PCM ground to the O2 sensing element, and the purple or purple/white wire is the signal wire you want to splice into. If you want to be able to choose either the left or right bank O2 sensor, run the signal wires from both sides to a single-pole/double-throw switch, then wire from the center pin of the switch to the guage. The A/F ratio gauges are really not much more than a psychadelic light show, unfortunately.

On the oil temp sensor, I've never really looked that close at my '01 Silverado, but on my LT1 I drilled a hole in the oil drain plug, and tapped it to match the correct sensor for the oil temperature. Simply unplug the sensor and the drain plug can be removed.

What engine do you have in the 3500?