All you're trying to achieve by turning on the intercom while testing the oxygen is to test if the mask microphone is working or not. So once you've established that nobody has turned off the bottles and that the regulator works (hence the pressure drop is below 50 psi) how you test that mike is your call in my opinion... Because I hate having my ear drums blown out by somebody else leaving the intercom on during the oxygen test I open the left flap and knock on the mask to test the microphone. Obviously you have to press the reset switch afterwards to switch back to the headset microphone.