Will a bad oxygen sensor(s) negatively affect gas mileage?

there possibly could be, have it scanned to see what code is related to check engine light BUT if it is a 02 sensor code you may want to check the data pid for the sensor and see how its reacting to changes in fuel control, a vaccum leak will fool a 02 sensor (giving a lean signal and computer will try to richen system)just as a rich fuel system-leaking/dirty fuel injectors, small hole in fuel regulator diaphram. (mechanical problem fooling sensor-computer don't know difference) you can make the system rich and lean and see how 02 sensor reacts to see if its the cause or the effect-hope this helps

Did you have engine light scanned for codes? A bad air/fuel ratio sensor(aka upstream oxygen sensor),will decrease mileage.If your mileage drop is only 1 or 2 mpg it may just be due to the switch by the oil refineries to their"winter blend" additives in the gas.Sensors aren't cheap,so get codes read first.