An oak lever is a lever which may be built in the Throne room of a player-owned house. It is used to operate the trap in your throne room (if there is one), and also controls challenge mode in the Dungeon. Left clicking on the lever will activate the trap in the throne room, while right clicking on the lever gives you the option to activate challenge or PvP challenge mode. The challenge or PvP challenge mode will end if you right click on the lever again. If you do not have a fiendish mechanism to activate, then the lever is useless on the trap.
Building an oak lever requires 68 Construction and 5 oak planks. It will give you 300 Construction experience.