Lysimachia ciliata - Fringed loosestrife. The most common being sold (and shared) is L. c. 'Firecracker'. I like this plant but it does spread quite aggressively by root & seed.

I was fooled to start with by what appeared to be tassel-like flowers. But now I think it’s Lysimachia ciliata ’Firecracker’ growing beneath an oak tree and festooned with the latter’s catkins.

eta typing while peren.all was already providing the answer.

Yes, we are buried in oak catkins right now!!!