There is no such feature currently.
What would you suggest this feature do?
As a work around I’d recommend checking a user’s topic list (via Profile → tab Activity) and then setting the notifications to Tracking or even Watching for each topic you find interesting.
As others pointed out, there are also downsides… OG should not be a popularity contest 
. All users should have an equal chance of having their new topic (or a new post) read.