Have you gone through the steps in Optus trouble shooting guide?
First off try a different USB cable and see if that makes a difference. If not, keep the phone charging overnight. If, in the morning, the phone is not turning on then ring the Optus mobile technical help section or go online and chat with them. You will probably get a return authorisation wherein a new phone will be sent to you and the courier will pick up the dud one. If you bought the phone at an Optus shop just go back to the shop and they will exchange it for you.