Knock Sensor vs. Oxygen Sensor?

The knock (a.k.a detonation, a.k.a. preignition) sensor is located on the cylinder head and detects premature ignition of the air-fuel mixture. It does so by sensing the sharp diesel-like combustion of the air-fuel mixture. BTW, knock occurs when the air-fuel mixture ignites spontaneously before the engine is ready, e.g. before the spark plug fires.

In a gasoline engine, detonation (knock) is a bad thing because the combustion is quite violent and causes very high cylinder pressures. Over time detonation (knock) can damage the piston or cylinder head and in rare cases, the cylinder bore.

Anyway, when the knock sensor detects knocking (detonation), it signals the ECU (Engine Control Unit) to retard the ignition timing to prevent the knock from occuring. Think of it as safety device for the engine.

The O2 sensor is located in the exhaust pipe fairly close to the exhaust manifold. It senses the amount of oxygen left over AFTER the air-fuel mixture has burned. The O2 sensor continuously informs the ECU of the O2 remaining. The ECU uses this information to determine if the air-fuel mixture is correct and uses it to adjust the mixture to the correct level.

A knock sensor is used to detect engine knock, specifically knock from detonation. It is made from piezoelectric crystal which generates an AC voltage when vibration or pressure acts on it. The onboard computer uses this signal to retard engine timing to eliminate detonation of the air/fuel mixture to prevent engine damage.

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An oxygen sensor is used to measure the oxygen content of the exhaust fumes in order to adjust the fuel mixture to attain as close to a 15:1 air fuel ratio at all times. The O2 sensor works like a half volt battery and it charges and discharges depending on the oxygen content. As the engine runs the sensor voltage is monitored and the computer increases or decreases the fuel mixture constantly. As a result the O2 sensor voltage fluctuates between .1 to 1 volt fairly rapidly when it is working properly.

An oxygen sensor is located in your exhaust manifold and helps control the air/fuel mixture of the engine. A knock sensor is threaded into the engine block and is supposed to sense when the engine starts to ping or spark knock and reduces the ignition timing accordingly. Two different sensors doing two different jobs.

A knock sensor actually senses the knock. It it a piezoelectric device which generates a small voltage under stress.

An oxygen sensor actually senses oxygen and has a voltage output proportional to oxygen present. it uses a zirconium ceramic bulb coated on both sides with a thin layer of platinum to do this

NcJay nailed it in the first answer. :)

I can only add that there is generally only one knock sensor, but could be 2-3 O2 sensors. On my car, there are 2 front heated O2 sensors, one in each exhaust manifold, and 1 rear O2 sensor, installed in the exhaust pipe just after the catalytic converter. For my car, a front one is more expensive to replace than the rear one.