I wouldn't ever recommend wood work tops around a sink.
If you must do it:
Use a tropical hardwood such as iroko as it is more rot resistant than oak; be warned it's also much harder, heavier and prone to burning when cut with power tools.
Have a sink that sits ON TOP of the worksurface (realise this is probably too late for you)
If having a Belfast type sink have a good lip all the way round the sink and cut a drip into the underside.
Seal all endgrain with a penetrating finish and multiple coats such as Rubio Monocoat (natural)
Agree on all the above. I really dislike undermount Belfast sinks with wooden worktops as every one I've seen has gone black. My solution was to not have a worktop around the sink at all. It's been in use a few months now and no signs of water marks so far 🤞
I wouldn't ever recommend wood work tops around a sink.
If you must do it: