on a former occasion vs once

on a former occasion

once

adv
  • Multiplied by one: indicating that a number is multiplied by one. 

  • One and only one time. 

  • Formerly; during some period in the past. 

conj
  • As soon as; when; after. 

How often have the words on a former occasion and once occurred in a corpus of books? (source: Google Ngram Viewer )