Why is my phone not switching on?

@dinesh26778 you need to tell us how customer care checked the motherboard. You also want to let us know what you have tried to fix this. Sure, it could be a bad motherboard but it could also be a few other things.

First you want to check your charger. Make sure it has the proper voltage. Check your charging cable to make sure the connectors are not bend etc. After that, check the charging port. Make sure there is no corrosion, debris etc. that prevents it from working.Clean it of any debris etc, use a few puffs of air to clean it out. Check the DC-in jack of your laptop. If none of this works, get an ammeter and see if your phone draws any amperage from the charger. After that, you need to check and see if your motherboard even gets power. For that of course, you need to open it up. Post Some good pictures of what you see. That way we can see what you see Adding images to an existing question

If this would be my phone and unless the "Customer care" actually did check the motherboard, my prime suspect would be the sub board (charging board) the flex cable going from the charging board to the main board and the battery. I would replace those before replacing the motherboard. Since iFixit does not yet have any guides for this phone use something like this video to work on yours.

If you do end up looking for a motherboard, then you will have to focus on a used one. New ones made by "Oppo" are not being sold. So ebay or Aliexpress/Alibaba may be your best bets. Careful what you buy so you do not end up with a locked or broken board and lose more money :-)

While you work on your phone, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has the same problem as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.

Repair is War on Entropy!