Hey @Amber786000 - Are you a current Optus customer? Can you please send through a private message confirming your IMEI number and unlock code?
Heya @Amber786000, I've replied to your PM 🙂
Please see our help and support page How to unlock your mobile device from the Optus network
It's unclear if you mean you purchased your Oppo (what model?) as a Prepaid phone from Optus?
This thread has a bit more info if it's the A73.