is "of course" a kind of preposition or a full adverb?

We all know there many kinds of preposition in english grammar such as:
Simple, Double, Compound, phrase, and participle.

Is "of course" a preposition or adverb?

Of course functions as a sentence adverb in most cases:

Of course I can help you with that.

Here's how Oxford Dictionaries defines the term:

Sentence adverb
Some adverbs refer to a whole statement and not just a part of it. They are called sentence adverbs and they act as a comment, showing the attitude or opinion of the speaker or writer to a particular situation.

See also: EL&U answer