I don't know what Osu is but I get the gist of the mechanic you're after.
For the functionality:
Just animate a float variable over seconds, e.g. 5.0s -> -5.0s.
Rate the users input in how close to 0.0s their tap/click is.
For the visual display:
A circular button with a secondary highlight "ring" that slowly scales down in size to be the same circumference as the button, using the above floats as a relative touchstone just make sure the visual "ring" perfectly overlaps the circular buttons outline at the same time the float is 0.0s.
Hope that helps.