I'm doing a CA18DET swap into my s13 and purchased my engine harness from someone in Australia. When I go to install the harness and do all of my rewiring for the swap i notice that there isn't an O2 sensor plug (or I don't recognize this kind of O2 sensor plug). This plug is in the same location that the O2 sensor plug normally is. Can anyone help me identify this plug? If the plug is actually not there can I just hard wire in an O2 sensor plug according to the FSM wiring diagrams.

simple one wire o2 sensor like on the early ka24e and ka24de motors.

You could always wire in a three wire..... Or a 4 wire if you have a stainless steel exhaust mani.

just hook white to this single wire and red and black to 12V and ground, respectively (I have a 3 wire)?

Yes, that would be ideal if you have the stock mani. Also you will wasn't to find a power source that is only on when engine is running.