If memory serves, the A-300's were introduced to CO
's fleet about 1985. The company was recovering from (its first) bankruptcy and needed cheap capacity fast. Airbus had some brand new A-300 'white tails' (planes built but without customers to buy them) collecting dust on the shelf - Airbus was eager to get rid of them - so CO
got the planes, probably on favorable terms.
Later on, after Texas Air Corp (then parent of CO
) bought Eastern, some of Eastern's A-300's were transferred to CO