All iPhones which have been purchased through Optus have been network unlocked. To complete this process, you'll need to either upgrade to iOS7 or restore in iTunes (best to back up first if you want to keep data once upgraded).
All new iPhones from Optus, running iOS7, will not be network locked.
Note: A device can only be unlocked once we receive a request from the original owner of the device or the Optus supplied Proof of Purchase that was provided at the point of sale. We cannot contact the original account holder on your behalf & second hand devices are not able to be unlocked without this info.