If the code said it was an O2 sensor then that's probably the cause. If Holden have agreed to foot the bill, just take it back to Subaru and make them replace the sensor rather than just clear the code.

If they are pricks about it, talk to Mao at Evolution R, he checked my code for free (what a nice guy eh?) then replaced it at about 30% less than Subaru wanted. You may have to google the code to see which sensor the code refers to but that shouldn't take long.

Oh, and congratulations on the purchase!