Onan is a product line, and Cummins Onan is a division of Cummins. It's a real company specializing in engines and power generators.
It plays a significant role in the books. Steve Sharp buys three old motors that run out to be from Onan. His neighbor and friend Abe turns out to be a retired head mechanic from Onan. Steve and Abe build up his business rebuilding and selling these and further Onan equipment, and it becomes a major line of S&S Enterprises.
Steve also hires Matthew (or Matt) formerly from Onan, to be his road technician and salesman.
Later, a group of Onan salesmen headed by their national sales manager, Robert Bigalow, tries to hassle Steve. Steve gets Onan's legal council on the line, and smacks Bigalow down. As a result, he gets a call from David Owens, President of Onan USA, and eventually large territory to sell Onan products into.