On your car, the O2 sensor is supposed to be mounted in the catalytic converter, or immediately in front of it, and there should be three wires going to it, from connectors at the upper right (as seen from the driver's seat) firewall, near the A/C hoses.

It is not unusual to find a plug, of the same diameter as the sensor, in the exhaust manifold or the downpipe, which could lead you to believe that the sensor is missing.

If your wires are indeed cut or broken, it may be that the sensor was replaced with a generic one that requires splicing of the wires, which was not done correctly, and the connections pulled apart.

If the O2 sensor is disconnected or missing entirely (and the hole plugged) it should turn on the check engine light and set a fault code. You will also notice rough running and poor gas mileage. In normal operation, the check engine light should come on when the key is turned to the on position, and go out when the engine is started. Is yours doing this? If the light never comes on, the previous owner probably removed the bulb, to hide the O2 sensor (or additional) problem(s).