Is this a "hot start" issue, or a "cold start" issue, or both?
If the engine isn't getting up to normal temp, and also takes a long time to get up to temp (approx 5 minutes gentle driving should see the engine at normal temp), then it is possible the thermostat is jammed part open, and needs replacing.
To test the gauge is working correctly, set the key to IGN, disconnect the wire onto the (single-pin) temp sensor, and ground it out on the engine - the gauge should go full-scale if it is working correctly.
NOTHING TO DO WITH THE O2 SENSOR! It only works for cruise conditions.