Run a fingernail across every mark on your old phone's screen before you answer the condition questions, and report it as a crack if your nail catches.
The exchange value on a phone's product page comes from your own answers about your old phone, and the courier checks those answers at your door. If the phone does not match what you said, or the check puts a lower value on it than your estimate, the courier cancels the whole order, new phone included. Your payment goes back to your original payment method, but you have to place a fresh order without the exchange. Overstating the condition is the most common reason exchanges fail.
A minor scratch is accepted. Any crack, dent, screen spot, lines on the display, loose screen or water damage fails the check, and so does a phone that does not switch on within 30 seconds or an IMEI or brand that does not match what you entered. Before delivery, remove every screen lock, charge the phone and connect it to the internet, because the courier runs a Cashify diagnostic app on it. On an iPhone, you also need to sign out of iCloud and turn off Find My iPhone.
Trading in certain OPPO models adds an exchange bonus on top of the phone's value, and the OPPO Exchange Program page lists the bonus rules.
Buying certain new OPPO phones adds an extra bonus, and the OPPO Store app and website show the current bonuses. One account can use these bonuses only once every six months, so if you plan two phone upgrades in that time, compare the bonus offered with each phone and use it where it is bigger.