Underway, he is the Commanding Officer's direct representative and is responsible for the safe navigation and operation of the ship for his watch. He ensures that any and all of the Captain's standing orders and night orders for the watch are carried out. As such he is responsible for all watch standers, even those who might be nominally senior to him. He may or may not be the conning officer but can take the conn from anyone but the Captain.
In port, he is responsible for controlling the brow and the comings and goings of all ship's company and visitors. He ensures that visitors are greeted properly and the appropriate person is contacted for ship's business. He is also responsible for the non-quarterdeck personnel on watch (roving patrol, engineering, anchor watch, etc.) Usually only junior officers stand OOD (in port) watches.
WWG1WGA
Garde la Foi, mes amis! Nous nous sommes les sauveurs de la République! Maintenant et Toujours!
(Keep the Faith, my friends! We are the saviors of the Republic! Now and Forever!)
LonePalm, le Républicain du verre cassé (The Broken Glass Republican)