what is obriens law?

Answer 1

I think the below sums it up:
O'Brien's Law:
Any "Surname's Law" formulation labelled as such by its creator is likely a facile exercise in self promotion.

It IS the name of an present Irish racehorse. which recently competed at Ascot.

There's no statute in the UK labelled as such and no law of physics or mathematics of which I am aware named as this. If you've heard of it, I suspect it's just a made up bit of hokey.

Answer 2

Never heard of it. Murphys Law is that "If anything can go wrong, it will".