I've been chasing this same problem on my 99 since March. Every damn time that light comes on, everything is running fine. And every time I hook a DRO up to it and pull the voltage readings while the engine is idleing, all the sensors check out just great and react accordingly with throttle input.
The only time it really flipped out was in April while I was at Spring Break at the beach. Was driving down the road and the light came on. Drove it to the nearest autoparts store and pulled the code, and got the same flippin one, P0138. On the way back from the store (happened to be an autozone) it started flippin out on the highway, wouldn't hold a gear, and wouldn't idle worth a damn either. Pulled off and let it sit a couple of hours on the side of the highway, and managed to limp it back to the beach house which was a good 5 miles away still. The next day I took it out and drove it up and down the island a couple of times, and even on the beach, with no problems. The CEL came on and off a couple of times on its own, but everything ran great, and it wasn't running rich at all.
Long story short I got it the 200 miles home and turned the light off (also pulled a code for the TPS which would explain the flippin out on the highway, as well as some tranny code that apparently can only be pulled with a special scanner for the AW4.). The o2 sensor code has come up only once since then, and the others havent come back up at all (still not sure why they did to begin with). Im not replacing the damn thing untill I can catch it acting up, to be absolutely sure its the sensor thats the problem. MPG hasnt suffered, and has actually got better since i put my flowmaster on (went from 14-15 all city to on average or 16-18 all city)