Use a multimeter in diode mode. Check pin 3 (VBUS). You should read 0.4V-0.6V drop to ground. If you read 0.000V (short), there is a shorted capacitor on that line.
Refer to the Component Placement page (usually the last 2 pages of the TOP PDF) to find the physical location of that capacitor on the board. oppo a15 schematic diagram pdf top
Open the TOP PDF and search for "Charging IC" or "BQ25601" (common OPPO charger IC). Use a multimeter in diode mode