as above just got a decat dowmpipe havent looked at it but dont think it got a hole for the sensor, does my car have one in it downpipe, i have a 05 blob ee wrx ppp.

The main O2 sensor is in the exhaust header, down by the drivers side.
There is a second sensor for fuel trim on the end of the downpipe, just before it goes into the centre section.

cheers, i thot there was a sensor, the question now is does the pipe ive got have a hole for one cant remember and its at work...