on the contraire/au contraire, on the contrary, to the contrary: The Weekend Edition—Common Errors in English Usage Entry for Friday–Sunday, September 19–21, 2014

on the contraire/au contraire, on the contrary, to the contrary
People who like to show off their French sometimes use the expression au contraire when they mean “on the contrary” or “to the contrary.” People who don’t know any better mix up French and English by saying “on the contraire.”

“On the contrary” is the earliest form. It means “it’s the opposite”: “I thought you liked sweet pickles.” “On the contrary, I prefer dills.”

“To the contrary” means “to the opposite effect,” “in opposition”: “No matter what my neighbor says to the contrary, I think it’s his dog that’s been pooping on my petunias.”

___________
The Week's End Extra from the Archives: “‘Who that?’ (And other non-errors)” (January 18, 2013).