think of vs think up

think of

verb
  • To create in one's mind; to originate an idea through thought. 

  • Used other than figuratively or idiomatically: see think, of. (to think on the subject of) 

think up

verb
  • To create in one’s mind; to invent. 

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