Antifreeze leaking from overflow tank

I live in Tucson, Arizona and today after work I cranked my '97 Dodge Grand Caravan and sat in the car with my wife for over an hour. During this time I had the car running with the airconditioning on. Someone walked by and told me that antifreeze was leaking from my car. I checked the temperature and it was in normal range. I popped the hood and could not find a leak, but saw that my overflow tank was almost overflowing. It was above the maximum amount. I also saw that there was some power steering fluid on top of the power steering fluid reservoir. The engine seemed really hot, even the hood prop was too hot to handle. Can anyone tell me what would cause this? After I let the car cool down for a while, the antifreeze in the overflow tank went back down to minimum. The van drove home fine without any problems. I should say that I think the 02 sensor is bad and the car seems to run lean. Any help would be great. Thanks!!!