location O2 sensor

well i have a 2000 civic si and i got a code 61 cel. its either o2 sensor or exhaust leak. the exhaust was checked for leaks yesterday and it has none. so i need to change the primary o2 sensor. Anybody know where the primary o2 sensor is? is it the one just behind the header? also the car stutters real bad about mid way through the power band. like around 3-4k rpm. feels like it almost skips a beat or something. its even worse when i have the air condition on. i have a cold air intake, skunk 2 intake manifold, header, exhaust. could it be not getting enough fuel?