Point of O2 sensor?...

[rhock99@epix.net (Ryan H)]

16V questions:  I thought the O2 sensor was to help the DPR determine fuel mixture, but Collin from TT told me to disconnect mine in order to see if it is working... Disconnected, the car doesn't run any different, so what's the point of the O2?
And while I have your attention, anyone know of what would cause low power; electrical related - i.e., after a little while of driving, power drops off badly.  Turn car off, turn car on, problem's gone for a little while...  Knock box and knock sensor have both been replaced.  Would coolant temp. sensor do this?  Coil?   
Thanks ahead of time... Ryan H.