Check out the CX forum, and you will see several threads about op-ups.

I'm AA plat (sapphire) and was op-upped on 3/4 flights in November on CX.

If coach is oversold, CX will first op-up their own MPC elite members, and then move on to other oneworld elites. They have been very good about recognizing my status.

They are generally discreet about op-ups, you will not be called to the podium like in the states, but the gate reader will have new boarding passes face down, when you go to board, if you've been op-upped, it will tell the agent who will then hand you your new boarding pass. Once I was given the op-up at check in.

good luck