It could very well be the CAT, and one way to check it is to kick and pound on it. Side to side and up and down. If it rattles or makes other noise, replace it. This test is a way to find a cat that'* obviously shot. Some failed cats may not make noise right away.
Is this the same O2 code that has been a phantom code for some other OBD2 cars here? Or am I thinking of another one?
We've had a few of those lately, the other being the P0440 (and related) check gas cap or evap. system leak.
You may want to get the car scanned, where the reaction of the rear O2 sensor can be checked. It should not react as quickly as the front sensor, and should have a relatively flat waveform. A back pressure test may help, also. Any performance problems as the car warms up?