I've come across the term "Ocean Sea" and wonder what it means, especially compared to simpler terms like "ocean" or "sea"? Is this a recognized term in English, either current or historical?
The Romans recognized two major seas: the mare mediterraneum or “sea in the middle of the earth”, and the mare oceanum, or “Sea of Oceanus” — Okeanos being the Greek name for and personification of the great “river” believed to encircle the earth.
In English, mare oceanus was translated Ocean Sea, as opposed to the Mediterranean Sea, and it was only about 1650 (according to the OED, volume 7, online here) that ocean started to appear alone as a noun designating the vast outer water, so much greater than mere local seas.
From Urban Dictionary, which, I assume is the OP's source, Admiral of the Ocean Sea
Noun (naval terminology): the universally supreme naval rank, awarded historically only to one seaman (Christopher Columbus) jointly by HRM Ferdinand and HRM Isabella of Spain, in recognition of his achievement in successfully navigating the Atlantic Ocean westward and (presumed at the time) the discovery of a shorter, less hazardous, and less expensive trading route to India from Europe.
"Upon his return from his first voyage to America, Christopher Columbus received the rank and title of "Admiral of the Ocean Sea" from Ferdinand and Isabella, which rank he holds in perpetuity."
Also supported by Wikipedia https://en.wikipedia.org/wiki/Christopher_Columbus
In the 1480s, the Columbus brothers proposed a plan to reach the Indies by sailing west across the "Ocean Sea" (the Atlantic).
By about 1484, Columbus presented his plans to King John II of Portugal. He proposed that the king equip three sturdy ships and grant Columbus one year's time to sail out into the Atlantic, search for a western route to the Orient, and return. Columbus also requested he be made "Great Admiral of the Ocean", appointed governor of any and all lands he discovered, and given one-tenth of all revenue from those lands.
In the April 1492 "Capitulations of Santa Fe", King Ferdinand and Queen Isabella promised Columbus that if he succeeded he would be given the rank of Admiral of the Ocean Sea and appointed Viceroy and Governor of all the new lands he could claim for Spain.
See also the book: "Admiral of the Ocean Sea": A Life of Christopher Columbus Paperback – 1 Oct. 1991 Samuel Eliot Morison