Connolly, 40, has created work for Classic FM, Eurostar and Pets Pyjamas while at Saatchis. He will move to O&M in September.
Connolly, who was named as one of Campaign's Faces to Watch in 1993, will work across the board on O&M's creative accounts as well as heading the agency's art department. His appointment is the latest move by O&M's executive creative director, Malcolm Poynton, to try to revamp the agency's creative department.
Poynton said: "It is fair to say that Ogilvy & Mather has been left wanting in this department in recent times and I hope that Brian's arrival will change things."
Connolly launched his career at Ammirati Puris Lintas as an art director in 1989. After a decade with the agency, he relocated to GGT and then Young & Rubicam before a two-year stint as a senior art director at BST.
He joined Saatchis in 1997, where he was teamed with the copywriter Paul Dominet. The pair recently won the monthly Aerial Award for their "Katie Lee" radio ad, created for Credit Card Fraud Protection.
Connolly said: "This is a fantastic opportunity for me, and they don't come along very often. What has impressed me about Ogilvy & Mather is that despite its size and the length of time it has been around, it almost feels like a brand-new agency."