O2 sensor

I have an 02 Mitsubishi Galant. For the past few months I have been driving around with the check engine light on. I went to the local Autozone and they told me my OBD port didn't work.. But when it came time to get inspected, the OBD came up with a bad Oxygen sensor. I promptly changed out the upstream sensor and the light went away. Then, a couple of weeks later, it came on again and once again it was the sensor that was diagnosed as faulty.

Now my question is: how common is it for an O2 sensor to go bad that quickly, what would be the cause, or could I simply have a sporadic problem with my OBD port?

Sometimes a loose/worn pin in the DLC can cause issues with scanner communication, but it shouldn't set a light or anything.

Do you have the code number by chance? Depending on the code, you could have many issues causing a faulty O2 signal other than the sensor itself.