My check engine light is on, went to auto zone got it tested and they said it was some kind of oxygen sensor, we'll I over looked it and went and got a oil change now my car won't go 20 mph. Please help
The oxygen sensors won't cause that. They simply help the Engine Computer fine tune fuel metering for lowest emissions, and the "downstream" ones monitor the efficiency of the catalytic converter(s). Basically there's three things to look at. There could be a lack of engine power from misfiring cylinders. You'd feel and hear that. Power could be down due to a jumped timing belt, but usually the engine will be hard to start too.
The exhaust system could be restricted, typically due to a plugged catalytic converter. Listen to the sound of the exhaust at the tail pipe. If you hear a steady hiss instead of the normal "putt-putt" sound, suspect a plugged catalytic converter. There should also be a good hard air flow from the tail pipe.
A disc brake could be dragging, but there will almost always be other symptoms or observations. Feel next to the two front wheels after driving a few miles. If one is real hot, have the brake system inspected.
Also be aware that diagnostic fault codes never say to replace parts or that they're bad. They only indicate the circuit or system that needs further diagnosis. When a sensor is referenced in the fault code, it is actually the cause of that code about half of the time. Your mechanic must check the wiring and do some electrical tests to rule everything out other than the sensor.