The communications officer, or COMMO for short, is a position aboard US Navy ships and larger US Coast Guard cutters. Other navies probably have analogous positions.
The COMMO is in charge of a ship's radio communications systems and frequently the ship's LAN as well. The position is generally held by a junior commissioned officer or a senior noncommissioned officer. On most ships, the COMMO works for either the Operations officer or the Combat systems officer, though on aircraft carriers and heavy amphibious assault ships, the COMMO might be considered a quasi-department head. (This is rather like the main propulsion assistant or navigator.)
Since the position involves working with a great deal of national security-sensitive information, the COMMO is required to hold a security clearance, typically up to the top secret level. If the position is held by a commissioned officer, that person will often, but not invariably, have some previous experience as an NCO. Usually this will be in the Navy or Coast Guard, but some USN and USCG officers were formerly enlisted in the Army, Marine Corps or Air Force instead.
A given COMMO typically holds the position for 2-4 years before transferring to another unit or being promoted out of the position.