The harder I work, the luckier I get.
As with so many quotes it’s difficult to pin down who said it. Whoever said it they were definitely on to something. There’s such a thing as simple luck, but in many cases we create good fortune by working hard, by investing time in ourselves and the things we do.
We create the circumstances by which good things can happen, and the more we do the more chances there are that we can find “luck” or good fortune.
So for a PhD candidate, success at the viva isn’t due to luck. Good fortune through the PhD, in writing the thesis and in passing the viva comes down to work: time invested in getting better as a researcher, effort invested in making something that wasn’t there before and energy invested in writing it up.
When you pass your viva it’s not through luck: you’ve done the work and made your own good fortune.