Mister O

Mr. "O" is the father of Buster Witwicky's best friend "O". He runs a diner in Portland, Oregan, called Mister "O"'s.

When the Transformers make their presence known on Earth, Mr. "O" puts on an Optimus Prime costume to attract reporters and customers to his business.

We were never told what the "O" stands for.

From Teletraan I: The Transformers Wiki, a Wikia wiki.