The Ontological Argument - valid or debunked?

[GrandizerII]

Possibly, meaning probability greater than zero?

That's certainly one. It's the standard very broad dictionary definition though Here's another more specific definition of one type of possibility

A logically possible proposition is one which is consistent with the axioms of a given system of logic.

And there are different definitions of probability too. For instance, there is Bayesian probability and frequentist probability

Yep, I had a good listen about that on YouTube once with a guy discussing probability types in the context of quantum mechanics.

And Bayes' theorem itself is something I studied a few years ago online when learning statistics and probability for fun.