HOW has signed a renewable contract with the John Lewis-owned supermarket chain to ensure the continuity of the existing campaign, which first aired in 1997.
However, CHI is expected to become involved in working on strategy and positioning assignments on Waitrose once the agency severs the last of its links with Safeway in November.
CHI has already been asked to work on the future direction of The Partnership, the umbrella organisation for Waitrose and the John Lewis department stores.
Its brief includes communicating The Partnership's benefits to staff and customers.
But Mark Price, the director of selling and marketing at Waitrose, said he expected CHI to carry out creative work on the Waitrose brand.
"CHI has a good understanding of our business and we intend to work with it," he said. "At the same time, we have signed a contract with HOW to produce work as we require it."
Price added: "We have been very straight with Chris and Ken, who have done fantastic work for us. But they are both in their late 50s and I have to consider the long-term position of Waitrose. Meanwhile, there will be a lot of work for both agencies."
Hoggins and O'Shea, the former creative partners at Banks Hoggins O'Shea/FCB (now renamed as FCB London), launched HOW in April along with Mark Wilson, who formerly ran the Waitrose account at FCB.
The agency has been hiring extra staff to service its Waitrose business.