Like subject complement, “Object complement” may be a noun, pronoun, adjective or clause, depending on the nature of situation. Actually, it is used to rename the noun it qualify to provide complete sense for object.
Meaning of object complement
When the object is person, then extra information become essential to give complete sense for expressed sentence. For this purpose, extra information is added in the form of noun, adjective, or even clause. Such extra information that immediately follows direct object, is called object complement.
•The team members declared him leader.
•They can’t force us to work like animals.
•He damaged my bike which I bought last year.
Object complement always follows the direct object of the sentence.
•My father gifted him a bicycle. In this example, “bicycle” is the direct object.
•My father gifted him a bicycle that is so stylish.
Some useful examples
•Teacher noticed students playing in the classroom.(complement is participle phrase)
•Last night, I saw him enjoying.(complement is participle)
•Gardner found boy stealing mangoes.(participle phrase)
•People termed leader incapable for them.(adjective phrase)
•Team members declared him leader. (complement is noun)
•They can’t force us to work like animals.(complement is infinitive phrase)
•They damaged my bike which I bought last year.(complement is relative clause)
•My father gifted him a bicycle that is so stylish.(complement is relative clause)
These examples show that the words like stealing, incapable, which, to work are used either to rename the earlier object or becoming something else.
They are participles, adjectives, nouns, relative clauses etc. used to describe direct object of the sentence.
On the other hand, adverbial complement is used to express time, place, purpose etc. for the action performed in the sentence.
For clarity, subject complement describes or identifies subject, object complement describes object, and adverbial complement indicates time, place in which action is performed.
I think this enough to answer the question what is object complement.