Why are electrons shared equally in oxygen, `O_2` but not in carbon monoxide `CO`?

We can answer this question by looking at the polarity of both molecules.

Oxygen, `O=O` is a diatomic molecule that is made from two atoms of the same element, and therefore, both atoms have the same electronegativity. Thus, `O_2` is a non polar molecule since the net dipole moment is equal to zero (`sumvecmu=0`).

In a non polar molecule, the electron density is equally distributed over the atoms making the molecule.

However, `C-=O` is a molecule that is made from two atoms of two different element, and therefore, the two atoms have different electronegativity. Thus, `CO` is a polar molecule since the net dipole moment is not nil (`sumvecmu!=0`).

In a polar molecule, the electron density is pulled by the more electronegative atom (which is oxygen), and therefore, the electron density is not distributed evenly on both atoms of the molecule `CO`.

Every element has a property called electronegativity (EN), which is the tendency of a bonded atom to attract electrons to itself. Bond character, such nonpolar and polar covalent, and ionic, is determined by the difference in electronegativities `(Delta"EN")`.

A `Delta"EN"` of `<0.4` indicates a nonpolar covalent bond. A `Delta"EN"` of `0.4-1.7` indicates a polar covalent bond, and a `Delta"EN"` of `>1.7` indicates an ionic bond. Some authors have different guidelines, so you should always check with your instructor or your book.
http://www.chem.wisc.edu/deptfiles/genchem/sstutorial/Text7/Tx71/tx71.html

The electronegativity of an oxygen atom is 3.44. Since two oxygen atoms make up a molecule of oxygen `("O"_2")`, the difference in electronegativity is `abs(3.44-3.44)=0`. A `Delta"EN"` of 0 means the bond is nonpolar covalent, and the electrons are shared equally.

The electronegativity of a carbon atom is 2.55. The `Delta"EN"` between carbon and oxygen is `abs(2.55-3.44)=0.89`. Therefore the bond is polar covalent and the electrons are not shared equally. The oxygen end will have a partial negative charge, designated by `delta^-`, and the carbon end will have a partial positive end, designated by `delta^+`.
https://en.wikipedia.org/wiki/Electronegativity#Pauling_electronegativity