Take a couple of breaths at the start as you sit down. Calm any nerves.
Take a breath whenever you get asked a question. Force a pause to compose your thoughts.
Mind starting to race? Take a few breaths. Need a second to make a note? Take one.
You will breathe throughout the viva of course. But remind yourself so you don’t get carried away with desperately answering questions. There’s time to breathe and time to think as you take a breath.