This year, the oak leaves were out well before the ash. According to the saw: “Oak before ash, in for a splash – ash before oak, in for a soak”.
So, what actually happened? We had 10 days of clear warm sunny weather, followed by a torrential downpour – presumably a soak.
The trouble with such predictions is that they are very imprecise, and can often be interpreted in a way that suits the interpreter! Five days after I noticed the different budding of the trees, it would have been true – today, it isn’t.
I think there may be a germ of truth in the saying. Ash is shallow-rooted, and oak has a taproot. So, oaks will generally fare better in dry periods than ash. But that isn’t the same as day-to-day weather prediction.