The assignment, to help drive sales across Europe, was made without a formal pitch and complements Weetabix's £15 million UK ad portfolio, which has been with the agency since November 2001.
During that time Weetabix has overtaken Kellogg's Cornflakes as Britain's favourite breakfast cereal.
BHO will co-ordinate the account, previously held by Bates in Belgium, out of London. Media buying remains with Zenith in Paris.
Weetabix ditched its "Withabix/Withoutabix" campaign when it switched its account out of Lowe and into BHO. Instead the company opted for a TV campaign under the theme: "Generating energy for everyone."
John Banks, the BHO chairman, said: "Breakfast cereal usage is very low across Europe and represents a huge opportunity for growth. The eating habits of a lifetime will need to be changed but it's not an impossible task."