Screen not getting on

@snehasishdas if your phone does not turn on it is hard to say if your battery is fully charged. I suggest that you start with the basic troubleshooting and then move forward from that point.

First check your charger. Make sure it has proper output. Then check your charging cable. Check to make sure connectors are not bend etc. After that, check the charging port. Clean it of any debris etc, use a few puffs of air to clean it out. If none of this works, get a USB ammeter and see if your phone draws any amperage from the charger. If it does not it is possible that this is a charging port issue or a bad battery.

To check the charging port, use a voltmeter and measure the voltage on your battery connector. Next plug your phone charger in. Now measure the voltage on your battery connector. That voltage will have to have increased. IF not then the issue is either the port or the charging circuit.

To open your Oppo phone and to work on it and since iFixit does not yet have a guide for this, use something like this video

If all of that checks out okay then the issue could be related to your power button flex or even the main circuit board. Let us now what you find out so we can try and help you further.