essay vs opus

essay

noun
  • A written composition of moderate length, exploring a particular issue or subject. 

  • A proposed design for a postage stamp or a banknote. 

verb
  • To move forth, as into battle. 

opus

noun
  • A work of music or set of works with a specified rank in an ordering of a composer's complete published works. 

  • A work, especially of art. 

How often have the words essay and opus occurred in a corpus of books? (source: Google Ngram Viewer )