I always use an Oxford comma:
I went to the shop to buy apples, pears, oranges, and grapes.
AND
I wanted to buy apples, pears, oranges, or grapes.
Our school apparently does not teach an Oxford comma :
I went to the shop to buy apples, pears, oranges and grapes.
But then what?
Is it:
I wanted to buy apples, pears, oranges, or grapes.
OR
I wanted to buy apples, pears, oranges or grapes.