Even if the carb is in great shape, all those vacuum driven smog devices are in unknown condition, and could affect the air/fuel mixture.
Also, how old is the O2 sensor? Back then the engine computer could not tell you when the O2 sensor was acting slowly. It could only give you an O2 sensor failure code if the O2 voltage was out of range.
See what it costs to replace the O2S if the mileage of the sensor is unknown. Otherwise, the air fuel mixture in normal conditions will be wrong.
Also, is your PCV system in good shape? pull out you PCV valve and shake it, listen to hear if it makes a metallic clicking noise. If it doesn't, buy a new PCV valve. Gases from the crankcase tend to shorten oil life if they are not vented properly.
I'm not sure how long an OCI should be for a Nissan Z24 engine.