Hey,

I believe that Optus is from Australia, so I know that almost all IMEI Unlock providers support it. As oodiyameed said, you can go to one of those IMEI Unlock providers (I do not know the company that he provided to you, you will have to check it out), and simply give them your IMEI number, your iPhone model and the Network you want to unlock. Usually in 1 to 5 days they unlock it (I mean all Unlock companies, not only the one that was mentioned).

You need to pay attention though to the status of your iPhone before making the order. Are you 100% sure that your iPhone is locked to Optus? Do you know its Blacklist status? Do you know if it has a replaced IMEI number? If this is your iPhone, then you should know all of these, but if you bought it from someone else, you must check all the above, because you may get scammed! You can perform an IMEI Full Check to that iPhone, by just knowing its IMEI number. This IMEI check will give you all the information you need, so you can proceed with an Official IMEI Order safely. Hope this helps!