1.Do you like apples, or oranges?
'or' is a coordinating conjunction that coordinates two clauses, of which the second is ellipted (part of it is omitted). Two short clauses comprise the sentence and the conjunction is adequate separation, therefore the comma is often dropped.
Do you like apples or (do you like) oranges?
2.Do you like apples, bananas, or oranges?
sentence #2 contains a series of elements that function as the object of the verb "like". The use of serial comma is necessary.
3.Which do you like, apples, or oranges?
Which do you like? (Do you like) Apples or oranges.
Which do you like: apples or oranges?
4.Which do you like, apples, bananas, or oranges?
Which do you like? Apples, bananas(,) or oranges?
Which do you like: apples, bananas(,) or oranges?
5.He or she is a student.
Student or Learner