Any holder of an office or of a post may bear the title officer. Generally, the word officer implies a rank, and degree of decision-making responsibility, higher rather than lower in a chain of command and reporting.
However, in some organizations that use the term, all but clerical workers are termed officers (in the U.S., police departments and the diplomatic service).
In military service, non-commissioned officer (NCO) (or petty officer in naval service) is a usually lower ranking officer with less responsibility (some sergeant majors, or sergeants major, are an exceptions with respect to both rank and responsibility) than any commissioned officer; their careers are also subject to different rules from those with commissions.
Officer is also a suburb of Melbourne, Australia.