Check Engine code says O2 Sensor but isn't

I have a 2001 Prizim with 87,000 miles. The check engine light came on. It was an O2 sensor code. The O2 Sensor was changed by Belle Tire. A couple weeks later it came on again and Belle Tire said to take it to the dealer. It was an O2 sensor code again. The dealer changed the Master Flow Sensor. A week later it is on again and it is reading O2 sensor. Any ideas on what it could be? Thank you

I had the same problem with my 1999 prism. There are 2 sensors on the car. One is in the rear near the gas tank.