Cat clicking is just heat expansion, a lot of cars do that. the O2 sensor is a common code and many things can cause it. For instance if you have an exhaust leak, a bad O2 sensor, the cat being bad. An easy way to test if its actually the cat is to get a laser thermometer, Harbor Freight has them for about $30+. The cats job is to heat the exhaust to burn off unburnt fuel. So a working cat will have a higher exit temp on the exhaust than the inlet. Usually by 50-100 degrees.