The charity has approached several undisclosed shops for credentials.
The review is part of a wider look at The Samaritans' marketing communications, which has also involved a brand audit by Wolff Olins. It is not known if O&M will repitch for the business.
The move is not said to be related to the recent cheating scandal in Campaign's Press Awards, in which a Samaritans execution, "Big Ears", was doctored.
Julie Alexander, the communications manager at The Samaritans, said: "If we were to review our agency it would be to do with our overall marketing strategy."
O&M referred all calls to The Samaritans.