Try open up back glass and check battery connector cable. It might be not properly attached due to the fall. I have same problem before with xperia z and i only need to make it intact back. You may need heatgun or hairdryer to open backglass
shelton is definitely right, it is the problem with digitizer. you can replace the digitizer only but the problem is lcd screen was glued to digitizer glass much like iphone screen, it is a hassle to remove the glue and requires technique so it is better to replace lcd fullset. if you wish to know how to open the phone try watching tutorial on youtube.
You need to buy a fullset lcd which contains lcd, digitizer and touchscreen glass. Water may soaked into lcd and caused that speckles. You also might want to check all flex cable connector for rust as water vapor may not exactly went out of the phone and caused that rust. Use soft toothbrush and rub it gently to avoid damage on those connectors. Often water damage caused damage to your screen, usb or audio holes that lead to motherboard and battery. If you say your phone is completely dry (better disassemble all parts first and let it dry more as there's might still vapor) and doesn't boot, try replace battery first. If lcd is powering on, you just need to see if it needs to be replaced
Your phone might have bootloop caused by software problem and it could be solve by hard reset. Hold volume up and power button at same time until screen shows oppo and small recovery mode text below. Just follow the wizard on screen to reset aka format and wipe cache so on. There are also way to reset using pc and you can find tutorial on how to do that googling oppo hard reset or unbrick oppo. Mine has same problem too but it cannot enter recovery mode or recognized by pc. So just waiting for expert help on this
Have you tried changing charger you used. If yes, you might have faulty charging usb port on the phone since it still occurs on new battery. Old phone charging port for can be found online especially on aliexpress. You can refer how to replace it by youtube tutorial