An object complement is similar to a subject complement, except that (obviously) it modifies an object rather than a subject. Consider this example of a subject complement:

The driver seems tired.

In this case, as explained above, the adjective "tired" modifies the noun "driver," which is the subject of the sentence.

Sometimes, however, the noun will be the object, as in the following example:

I consider the driver tired.

In this case, the noun "driver" is the direct object of the verb "consider," but the adjective "tired" is still acting as its complement.

In general, verbs which have to do with perceiving, judging, or changing something can cause their direct objects to take an object complement:

Paint it black.

The judge ruled her out of order.

I saw the Prime Minister sleeping.

In every case, you could reconstruct the last part of the sentence into a sentence of its own using a subject complement: "it is black," "she is out of order," "the Prime Minister is sleeping."