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I have 1990 Toyota GT4, how important is it to replace the O2 sensor? I believe the sensor is still the original.

If there are no indications of the oxygen sensor giving problems, then leave it alone.

Changing for no reason could be an expensive exercise especially if a genuine part was used.

Plus, we get to hear of problems where aftermarket sensors are fitted and they give nothing but trouble. Wiring connections are often different which in itself can lead to problems trying to make a decent connect between the main harness and the sensor.

Trying to remove the old O2 sensor can also be a nightmare on some vehicles.

Don't go there unless you have to. If the engine management warning light is not staying on, and the engine is running OK then you have no need to worry.