a lot vs oftentimes

a lot

adv
  • Often; frequently. 

  • Very much; a great deal; to a large extent. 

pron
  • A large amount. 

  • Many things, much. 

oftentimes

adv
  • (elsewhere, archaic) Frequently; often 

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