O2 Sensor Help

1995 4runner 3.0

So my 4runner was straight piped when I bought it however I've started to wonder about the fuel/air ratio and if it is actually running "well". The other day I pulled out the spark plugs and they didn't look great. I've also been told by the guy at the shop that even though its a 3.0 slow it has much less punch than others and seems to be lagging power. I'm wondering if its because both O2 sensors have been deleted and its running on open loop? I've heard that the upstream o2 sensor (before cat) manages the mixture.

Does anyone know if I install an o2 sensor upstream and leave out the downstream o2 sensor, if the truck needs both for the one to function? Or will the upstream sensor function without the downstream?