For warranty, u can just use the official receipt that shop issue. Service center once recognise it's one of the 10 official reseller, they will acknowledge it.
If you bought a used set from bro here, just make sure there is an official receipt from the shop he bought from
Alternatively, if u lost the receipt, go back to the shop u bought from and get one from them.
Oppo is using a traditional way of filing warranty. But as there is strictly only 10 reseller, so long it came out from their shop, u are safe.
Anyway, so many oppo set out there, if you opt for a change and didn't receive any notification, guess they might be having a headache or might miss out some of you. Just give them a call before u head down.