An oar is an implement used for water-borne propulsion. Oars have a flat blade at one end. Oarsmen grasp the oar at the other end. The difference between oars and paddles are that paddles are held by the paddler, and are not connected with the vessel. Oars generally are connected to the vessel by means of rowlocks or tholes which transmit the applied force to the boat. In this system (known as a second class lever) the water is the fulcrum.
Oarsmen generally face the stern of the vessel, reach as far as they can towards the stern, and insert the blade of their oar in the water. As they lean back, towards the vessel's bow, the blade of their oars sweeps the water towards the stern, providing forward thrust – see lever.
For thousands of years vessels were powered either by sails, or the mechanical work of oarsmen, or paddlers. Some ancient vessels were propelled by either oars or sail, depending on the speed and direction of the wind (see galley).