There’s no luck with the viva. No trick or superstition to rely on for success. Instead, it’s all on you.
What you did, what you know, what you can do.
None of that is due to luck either. There could be good fortune – when hard work pays off – and you achieve something that was uncertain, but there’s no simple luck.
There’s nothing that just gets you through – and nothing that simply, randomly, unluckily stops you.
You worked for your success. That work continues to help you through the viva.