1994 Buick Century poor performance

My 1994 Buick Century wagon has 156K. Last week, while rainiing, I noticed that the vehicle would start to bog down while driving the interstate at 65mph. As I started to climb a hill, the more I accelerated, the more the vehicle slowed. If I cut back on the gas, the vehicle ran normal. This would only happen intemitantly and only when raining, I think. The code reader said P0131 error, ox2 sensor but I was wondering if the fule filter or pump can be bad or my wiring? It also hesitaed really bad from a dead stop, hot or cold. Where are the ox2 sensor and filters located?