I don't see any similarity???
(of course spelled ofcourse) is making a compound word out of two words in a way which is not done in English (at least, I can't think of any words where of is used as a prefix), and the resulting word is not a valid English word and so this mistake will be caught by any spellcheck.
Often?
Though it is not a compound word, since it is just an extended form of oft, as far as I know. But looks very similar to what you can't think of...
Often is not derived from 'of ten', it is, as you rightly point out, related to oft. Thanks for emphasizing my point for me.