The adverb o'clock is one we tend to use without giving it much thought. Maybe you're more likely to say, "I'm sleepy, but it's only eight," and in this case the o'clock is implied. It's a shortened form of the now obsolete phrase of the clock, from the Middle English of the clokke. You might also use it when you're describing a direction by referring to an imaginary clock: "I saw the deer run by right there, at twelve o'clock."