Correct spelling, explanation: of course is an adverbial phrase that comes from the 16th century of the ordinary course; something in a natural order. As a phrase it should be written separately – these are two different words, which never got to be written as one. Therefore ofcourse is incorrect, as the phrase should be spelled separately – of course.

Definition of of course:
the adverbial phrase, in regular order, obviously
-Could you please close the window? It’s getting cold. –Of course!
Of course, our staff is here to answer any of your questions.

Incorrect spelling, explanation: ofcourse is an incorrect form because this is a phrase, so all words in it should be written separately – of course. The phrase was previously used as of the ordinary cause, later it got shorter. Therefore of course is the correct form.