We have #4+2xx6=16" valence electrons"# to distribute over 3 centres. Carbon is considered to be the central atom (this is typical in VESPER; the least electronegative element is considered to be the cental atom).
Each oxygen atom must be surrounded by 6 electrons to be considered neutral, and there such a number of electrons: 2 bonding electrons, and 2 lone pairs on each centre.
For #O=C=O# there are thus 2 lone pairs associated with each oxygen centre. There are 4 electrons associated with the central carbon. Each element is therefore neutral. Given that there are 2 regions of electron density around #C#, the molecule is linear, i.e. #/_O-C-O=180^@#.