Board Thread:Support Requests - Getting Technical/@comment-4839035-20130124164503/@comment-5209426-20130124194227

That's the expected behavior of the Monobook skin, it's the same as on Wikipedia. Monobook inherits that from the default browser behavior, blue links which turn purple after having been clicked have been standard since 1995.

Oasis overrides the browser's default colors but does so without setting a visited style. You can provide your own visited style in Monobook to hide it if you really want to: