The company signed CHI without a formal pitch, and will launch its campaign in September, backed by a multi-million-pound spend.
CHI has close ties with Waitrose, having worked with the former marketing director Mark Price, who is now the Waitrose managing director.
Jon Rudoe, the head of trading and marketing at Ocado, said the company wanted to update its creative message with the view of appealing to a broader audience.
He said: "We are working on communicating what we do and how the service continues to improve as we grow up as a business. For the first time, that communications mix will also include TV advertising."
Ocado's last agency arrangement was with Ogilvy Advertising, which ended more than a year ago. Since then, it has handled advertising in-house, with some radio ads reworked by Comesitdo.