You don't need to deactivate the O2. In fact you can't AFAIK.
You can disable it from generating an error code though, but that's not nesesary.
You will need to disable the CAT efficiency error code which will at some point come on after the decat due to rear O2 sensor and front O2 sensor discrepancies, due to no cat.
With the cat removed the TD04 on the 2.5 WRX will boost like a bastard and will need some heavy BCS duty adjustments to get it back under control.
The time and effort I would expend on getting a decatted car running right isn't far off the time spent on a car with injector and turbo changes.
The longest part of an mapping session is getting the boost under control all the while running safe fuelling and timing. Only then can you focus on fuelling and timing optimisation.
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