An object complement is an noun, pronoun, or adjective which follows a direct object and renames it or tells what the direct object has become. It is most often used with verbs of creating or nominating such as make, name, elect, paint, call, etc.
We know there is a difference between calling Mayor Williams and calling Williams mayor or painting a red door and painting a door red. When the word follows the direct object and it tells what the direct object has become, it is the object complement. Sometimes people call it an objective complement. The italicized words on this page are object complements.