An oar is an implement used for water-borne propulsion. Oars have a flat blade at one end. The oarsmen grasp the oar at the other end. What distinguishes oars from paddles is that paddles are held by the paddler, and are not connected with the vessel. Oars generally are fastened to the vessel.
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 trireme and bireme).