Service Engine Soon - P0141

I have an Service Engine Soon light that comes on and off. Autozone pulled the code P0141, which is a heated O2 sensor on Bank 1 Sensor 2 (after cat. converter). The van is 171K miles, 1999 Plymouth Grand Voyager SE

Does it mean the oxygen sensor needs to be replaced? Where is this located? What's the benefit of getting the OEM vs. Walker or other generic brand? Can it be done by myself (I don't have jack stand)?

I heard the after catalytic O2 sensor is not really important in the air/fuel mixing, but of course, it's important to monitor the emission.

Thanks.

O2 sensor heaters are there to quickly heat the sensors up to operating temperature just after starting to reduce pollution while the engine is warming up. Once the engine is up to temp the heaters don't serve a purpose. Only three reasons to change it; 1) the service engine light is an automatic failure during emissions testing, 2) you are environmentally minded,3) you start seeing your mileage decline or it starts to hesitate or is sluggish.

Here's how: http://www.autozone.com/az/cds/en_us/0900823d/80/1a/3f/7b/0900823d801a3f7b/repairInfoPages.htm

Thanks for the detailed response.
I'll keep monitoring the mileage then...