A general term referring to the transformation items used in Kamen Rider OOO. The components that make up a Greeed and as a representation of desire and greed, O medals are classified in two categories: Core Medal and Cell Medal.
Core Medals are what makes up a Greeed and comes in three types: Head, Body, and Legs. These medals are what Hino Eiji uses to transform into Kamen Rider OOO.
Cell Medals are the manifestation of human desire and make up the body of a Greeed and Yummy. These are used for the technology developed by the Kougami Foundation as well as the transformation item used by Kamen Rider Birth.
For another explanation, the character Ankh explains that the medals are like a popsicle in the sense that Cells are what makes the ice cream part of a popsicle and Cores are the stick to keep it together.