It’s possible your research went down a path you didn’t choose. Your supervisor insisted. You followed. Whatever that meant for your research, you still disagree with the approach or idea now.
…That’s it. You disagree. That’s OK. Disagreement by itself is not a problem.
Did it stop you doing your research? Did it remove possibilities? Did it help the research but was tricky to do? Was it a tough conversation?
What’s the real problem?
If there’s something to explain in your viva as a result, you might want to think carefully about the words you use. If there is bad feeling, think about how you express that if you want to – but who would that help? You can still say you disagree with something from the course of your research.
Explore and explain. If there was disagreement with your supervisor about something, it would be good to reflect before the viva so you have key points to reference if you need to talk about it.
Disagreement by itself is not a problem: the situation might be, but the disagreement itself is not.