CO has a triple bond, which is composed of one sigma bond and two pi bonds. Pi bonds are formed using one p orbital from each atom, so the carbon atom must have two unhybridized p orbitals in order to form two pi bonds. The other s and p orbitals that it has are hybridized together to form two sp orbitals. One of the sp orbitals is used in the sigma bond and the other holds the lone pair.
