Formal Charge in Coordinate covalent bonds
The coordinate covalent bonds are still counted as 1 electron, since the pair is shared between both atoms. That means the Carbon will have a formal charge of -1 and Oxygen a formal charge of +1 on Carbon Monoxide.

Carbon FC: $\ce{(4 valence electrons on neutral atom) - (1*2e- from lone pair + 3*1e- from covalent bonds) = -1}$

Oxygen FC: $\ce{(6 valence electrons on neutral atom) - (1*2e- from lone pair + 3*1e- from covalent bonds) = +1}$