If you're not losing coolant, its likely to be condensation - a normal by-product of combustion. Engines that do short trips where the exhaust doesn't get hot enough, water can build up. In some cases, with mild steel exhausts, it can completely rust them out. It becomes more noticeable on cold mornings due to the temperature difference between the exhaust gas and the air. I have seen tech training cars (spend their entire life idling for short duration) spit out litres of water when given a decent rev. Don't be too quick to blame the O2 sensor!
