Keenleyside joins the agency from Claydon Heeley, where he held the role of head of copy.
At OgilvyOne Keenleyside will be responsible for overseeing the creative on the agency's British Gas and Zurich accounts, reporting directly into executive creative director Colin Nimick.
Keenleyside has worked in the advertising industry for more than 25 years, at agencies including Grey, TBWA and DraftFCB.
He has also worked as an associate creative director at Wunderman, working closely alongside Steve Harrison.
Nimick said: "Ross is a key appointment for us, someone who's going to play a big role in our drive to constantly improve the quality of our creative output."
At the same time, OgilvyOne has also appointed the Pamela Conway, the former client services director at Leo Burnett, as a managing partner on the British Gas account.