Q-Eng

usage note

1, going away from somewhere
I'm off to work. = I am going off to work.
I'm off to school.

2, not being at the place where I normally am.
I'm off work. = I'm not at work. (perhaps I am ill)
She's off every Tuesday.

3, unsatisfactory or disappointing
Everyone has their off days.
That pitcher is having an off day.
I had an off day.
obviously the market is having an off day, and this is a marvelous opportunity for you to double your stake.

4,
An off day : A day when things go poorly.
A day off : A day when I do not have to go to work.


source:
http://www.listen-to-english.com/index.php?id=540
http://www.natcorp.ox.ac.uk/
http://www.thefreedictionary.com/off
http://eow.alc.co.jp/off/UTF-8/