Why is the electronegativity of oxygen GREATER than that of carbon?
Well, basically because oxygen has a larger nuclear charge than carbon: i.e.
Electronegativity is defined as the ability of an atom in a chemical bond to polarize electron density towards itself. There are various scales, of which the Pauling Scale was the earliest, and is still widely used. Pauling used parameters such as ionization energies, and atomization energies, and then normalized the scale, so that it had small units.
Electronegativity increases across the Table from left to right as we face it. It decreases down a Group. This is reasonable in that incomplete electronic shells shield nuclear charge very ineffectively, and this results in the decrease in atomic radii from left to right across a Period as we face the Table, and the increase in atomic radii down a Group.
And because oxygen has greater nuclear charge than carbon, it tends to polarize the electron density of the