Press and hold the Power button until the power menu appears
Tap Power off
Tap Power off again if prompted
If the screen is frozen, press and hold the Power button and Volume Up button together for about 10 to 15 seconds until the phone turns off
If your Oppo phone has a side fingerprint or power key, press and hold that key until the power menu appears
If the phone does not respond, let the battery drain completely and then recharge it