Could have/Could Of

We often find the preposition of mistaken by the contraction of have along with conditional verbs such as Could. A reason for this is that the pronounciation of both, especially in certain accents, can make them sound alike.

Examples:
He could of gone home earlier.

I could've had more to drink.

However, the correct expression is Could Have as it is formed by Can + To Have.

Source: Oxford Dictionaries