In As You Like It, Oliver is the oldest son of Sir Rowland de Boys.  Orlando is one of his younger brothers.  In those days, the oldest son would get all the father's property as an inheritance and would be the head of that generation.

Because of that, Oliver is in a position to do bad things to Orlando.  He does not allow Orlando to get an education, for one thing.  He hits Orlando and orders him to leave their home.  And he convinces Duke Frederick that Orlando is a danger and needs to be removed.  Finally, he goes so far as to plot with the Duke's wrestler to break Orlando's neck.

All of this forces Orlando to flee into the forest.