What are the significant differences between Oliver and Orlando?
One of Shakespeare's themes in As You Like It is the conflict between the natural world and natural man (the Forest of Arden and Orlando), on one hand, and the political world and corrupt man (the court and Oliver), on the other.
Perhaps the clearest articulation of the essential differences between the two brothers is provided by Oliver himself:
. . . for my soul . . . hates nothing more than he [that is, Orlando]. Yet he's gentle, never schooled and yet learned, full of noble device, of all sorts enchantingly beloved, and indeed so much in the heart of the world . . . that I am altogether misprised. (1.1.154-160)
Those attributes that make Orlando beloved--gentility, wisdom, true nobility--are exactly the character traits that Oliver lacks. In short, Oliver sees in Orlando everything that he wishes he were but is not.
From Orlando's point of view, Oliver, who, as his brother, should treat him lovingly, has betrayed his familial obligations and, more important, betrayed their father:
[Our] father charged you in his will to give me good education; you have trained me like a peasant, obscuring and hiding from me all gentleman-like qualities. (1.1.62-66)
Oliver's ignoring of their father's express wishes indicates both how envious he is of Orlando's natural goodness and the extent to which Oliver has become politically corrupt. As long as he keeps Orlando uneducated and poor, the less he has to fear (as he perceives it) from a potential threat to his political power from Orlando.
The differences between the two brothers are stark: Oliver is the embodiment of a corrupt political leader--envious, hateful, unloved--while Orlando symbolizes natural goodness, intelligence (without the benefit of education), and nobility (without being "noble"). Together, the two brothers represent two worlds that continue to clash throughout the play--the corrupt court and the natural world.