Oreo O's was a cereal made by Post, which consisted of Oreo flavored O's with white sprinkles.
A variation of Oreo O's called Extreme Creme Taste Oreo O's contained Oreo filling flavored marshmallows..
The cereal was launched in 1998 and discontinued in 2007. They no longer make the cereal because Post and Kraft are no longer co-branding. Post owns the recipe to the cereal, while Kraft owns the rights to the Oreo name. According to a recent news article Post and Kraft have agreed to start production of the cereal again, with the rumored availability date around present day to February 13, 2013. However no official announcement by Post or Kraft has been made as of yet.
Read more about Oreo O's: International Availability