"To avoid confusion vs os

"To avoid confusion

os

noun
  • An opening or entrance to a passage, particularly one at either end of the cervix, internal (to the uterus) or external (to the vagina). 

  • Synonym of bone. 

  • An osar or esker. 

How often have the words "To avoid confusion and os occurred in a corpus of books? (source: Google Ngram Viewer )