What is the formal charge of carbon atom in the lewis structure of carbonyl sulfide (##OCS##)?

To a first approx. the Lewis structure of carbonyl sulfide is the same as that of carbon dioxide. The formal charge of ##C## is thus ZERO.
Given an ##O=C=O##, or ##S=C=O## Lewis structure, there are 8 bonding electrons around carbon, 4 of which this atom claims. And thus with 2 inner core electrons, the 6 electrons balance the nuclear charge to give a neutral carbon centre. Of course all the atoms in this molecule are formally neutral.
The ##C=S## bond length is approx. ##1.6xx10^-10*m##, the which is shorter than a regular ##C-S## bond, approx. ##1.8xx10^-10*m##.