of, off

Off, meaning "not on" or  "not in operation"--
This cake is not done. The oven was off.
There is no cake. The baker took the day off.
Turn off the light.
Turn off the highway when you see the bakery sign.
Off is also used to mean less, or the absence of--
They are offering ten percent off  everything in the store.

Of is a preposition--it links two words or phrases, and tells of their relationship to each other--
May I have a piece of cake?
This is the best of all the bakeries.
There are two more miles of highway before we get to town.
This bakery was voted the best of all.
Don't forget to make a note of its name.

Now that you know that, you can say--
"The bakery is offering ten percent off every box of cakes".

Note--of and off may be used in a way that seems the same--
10% "off " means subtracting 10% from the total price--so if the price started out to be $100, and you subtracted 10%, meaning $10, the net price would be $90.
10% "of " means just that--10% of $100 is $10.
Suppose you were offered a rebate of 10%--if the original price was $100, the rebate would be $10, making the net price $90.


Not to be confused with often, meaning frequently, or--
oven, the cooking box or chamber usually located under the stove.