oath vs undertaking

oath

noun
  • A statement or promise which is strengthened (affirmed) by such a pledge. 

  • A light, irreverent or insulting appeal to a deity or other entity. 

  • A curse, a curse word. 

  • A solemn pledge or promise that invokes a deity, a ruler, or another entity (not necessarily present) to attest the truth of a statement or sincerity of one's desire to fulfill a contract or promise. 

undertaking

noun
  • A promise or pledge; a guarantee. 

  • That which is undertaken; any business, work, or project which a person engages in, or attempts to perform; an enterprise. 

  • The business of an undertaker, or the management of funerals. 

  • The act of one who undertakes (in either sense). 

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