Change your plugs and wires. Cheap maintenance. Then replace that t fitting your put a clamp on. Check the bolts on the sensors on the Throttle body and make sure they are tight and the TB is tight on the SC. If any of your vac. line rubbers look cracked or corroded replace them.

As for the o2 Sensor yes it can cause bad fuel mileage and lean conditions but most of the time bank 1 lean is a vac leak.