Recently we moved where there is no Optus network coverage. My daughter has an Oppo phone and was using the Optus network, so I enquired about unlocking the phone so we could switch to a telstra provider. I paid $80 for an unlocking code, this didn't work, so I called Optus and was told by a useless tech that it is impossible to unlock an oppo unless we had an account with optus as such, not use the network!, So why isnt this stated when we put in the imei details in order to get the unlocking code! false advertising! I have put a complaint in with Optus, I have spoken to Oppo who also take no responsibility, apparently, they have made their phones so its unable to permanently unlock from Optus, Ive had no reply as a resolution from either, so now I am going to go through ACCC, so consider this your letter Optus. Its only $80 but its a lot to us atm and its the principle, don't take someones money if you are unable to provide the service you offer! Simple really!
Given you've already logged a complaint with Optus (assuming via www.optus.com.au/complaints ) then let this process run its course. They're pretty good and solved a problem where I kept getting various outcomes for a shipment.
Did you put the other SIM into the Oppo phone, and then use the unlocking sequence provided by Optus?