The 27 year old, who has shed 50 pounds since 2009, credits the dairy drink with helping her keep a trim figure.
She says, "I never thought I'd say this but I really do believe that breakfast is the most important meal of the day, and I always make sure I have mine with low-fat milk.
"I learnt a lot about healthy eating during my time on Dancing With the Stars in the U.S. and have been eating breakfast every day since. It just gets my day off to the best possible start."
In the campaign poster, a red satin gown-clad Osbourne wears her platinum hair in curlers as she munches on a bowl of cereal.
Singer Kelly Rowland and actors Rupert Grint and Ryan Reynolds are among the other celebrities who have starred in the British equivalent of America's long-running Got Milk? campaign.