first officer vs number one

first officer

noun
  • The executive officer, second in command of a naval vessel. 

  • Air Transport Auxiliary rank corresponding to Flight Lieutenant in the Royal Air Force. 

  • Women's Royal Naval Service rank corresponding to Lieutenant-Commander in the Royal Navy. 

  • The copilot of an aircraft. 

  • The chief mate of a merchant ship. 

  • The most senior civil servant in the United States Foreign Service, with responsibility for running an embassy under the authority of and reporting to the ambassador. 

number one

noun
  • A first lieutenant. 

  • The main goalkeeper of a team, so-called because they wear the number 1 on the back of their kit. 

  • The most important person, the one who is in charge. 

  • Urine; urination. 

  • A large town where theatrical performances may expect to achieve success. 

  • Used other than figuratively or idiomatically: see number, one. 

  • Oneself, being considered foremost, as by an egoist. 

  • Someone who is top of a ranking, who is ranked first. 

  • The batsman who opens the batting. 

  • The single that has sold the most in a given period. 

adj
  • First; foremost; best, often used after its headword. 

How often have the words first officer and number one occurred in a corpus of books? (source: Google Ngram Viewer )