have a look vs onlook

have a look

verb
  • To examine, to observe. 

  • To search for (something). 

onlook

verb
  • To look on or look at; watch; observe; view; regard. 

noun
  • The act of looking on (something); observation. 

  • One's perspective or outlook. 

  • That which is looked at, regarded, or considered. 

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