Power and volume buttons sunk inside the body of Oneplus 3

The same thing happened to me a few months ago. The power button went first, and then the bottom volume button some weeks later. I changed my phone battery today and discovered the little plastic bits had basically degraded and crumbled. So now even though my phone is charged up, I can’t get it to turn on and I haven’t seen anything on any forums about what to do about this specific problem. You posted this since 2019, have you had any luck since then? Buying a new phone is not an option for me.

There is an inherent fault on the case buttons of the OP3/3T. The Rubber plungers that make contact with the internal ribbon cable button fails over time, it goes hard and brittle, then crumbles. The case buttons need replacing. To do this you require to remove the back case (follow the instructions). Running down either side of the case is a granular adhesive strip. Using tweezers, peel this back carefully until you go past the case buttons. You will see a tiny metal spring clip that tensions the case button in place. Using tweezers again, gently lift that off. You can then push the button from the inside and it will fall out. Then replace with a new button in reverse order.