That’s what the viva is all about. How good is your research? How well did you do it? How well do you know it and your field? And just how good are you?
It’s a test that doesn’t require perfection to pass. Preparation will help you get ready, but remember: you can’t have got to submission by being merely lucky, and you can’t have done the work unless you were good.
The viva might test you, but your talent will help you succeed.