1999 Toyota Camry O2 SENSOR

I have a Toyota Solora, When I bought it around 5 years ago the check engine light came on. I had it checked out and it was my O2 sensor. I had it replaced by a friend and my check engine light came back on. I had a mechanic look at it and he said the codes just needed to be reset all is hooked up right. The check engine light came back on. I have not messed with it for about 4 years. I get around 14 miles to a gallon because of this. Recently I brought it in to have it looked at again at a different place they said the same thing everything is fine the codes just need to be reset. Of course my engine light is back on again! I am so frustrated. What could this be? A problem with my computer maybe? Any help with my appreciated :)
Thanks!


edit: I will try to do that soon :) Thanks also I was reading some of the other answers. Could it be my mass airflow sensor and not the O2?