Some aggressive demo builds disable recovery access. In that case, you need a PC and OPPO’s flashing tool (see Bonus section below).
If the phone requires a demo mode password or is managed by a store, contact OPPO customer support or the retailer. They can provide a special unlock code or tool. oppo demo mode remove
Dial the following code exactly: *#8011# (On some older models, try *#8778# or *#36446337# ). Some aggressive demo builds disable recovery access