The agency won the business after a four-month pitch process, which culminated in a direct shoot-out between Arnold and WCRS&Co.
The process was handled by the AAR and the appointment was made by Philip Price, the head of brand marketing for P&O Cruises.
Arnold will now be responsible for overseeing all of P&O's marketing and brand strategy, working closely with the cruise-line operator's media planning and buying agency, Vizeum, as well as its in-house marketing team.
P&O called the review of its advertising in January, after splitting with Dare, its incumbent of ten years, citing "different priorities going forward".
Dare, through MCBD, won the P&O account in March 2001, and retained it again in 2008. During its time on P&O, the agency created numerous campaigns, promoting P&O's ability to help travellers explore new places.
Earlier this year, Dare launched a TV ad celebrating the joys of travel, which introduced the strapline: "Discover a different world."
Price said: "Arnold KLP demonstrated a real grasp of our communication and channel needs. We have been very impressed with their ideas and creative thinking and are looking forward to working with them."