It scooped the business, which spent £1 million above the line last year, after a series of strategic presentations involving a number of undisclosed agencies.
The agency will develop the advertising for Tilda's mainstream grocery business, which is focused around the dry rice market.
Tilda will continue to work with the Leicester-based agency JFW on its marketing to ethnic consumers and with Jellybean for its food-service business.
O&M's first work is expected to break before the end of the year and will be focused around a TV campaign.
The account was previously held by St Luke's, which won the business back in 2001, following a pitch against JFW.
It is not known whether the agency was invited to repitch.
The most recent campaign for Tilda targeted Asian families and ran on Asian TV channels such as Prime and Zee.
Media planning and buying was not affected by the review and will continue to be handled by Media Planning Group.