Help with OBD II code PO304?

Hi, this code keeps coming up and I'm not sure what to do next. I have a 2001 Ford Taurus OHV 3.0 engine. I have already changed the coil, the spark plugs, and the plug wires. Does anyone have suggestions on what I would do next? I would really appreciate your help, because it keeps coming up. Someone told me to change the oxygen sensors, is this a good idea?