It possibly as simple as typical sibling rivalry. In 'As You Like It' Shakespeare is taking a theme which is so common in very many families and expanding it to encompass and magnify that on a huge scale - the social ramifications of taking sides in a whole country when ruling brothers are enemies. It's possible that Oliver is plain jealous (perhaps Orlando is better-looking as we know that Rosalind takes a shine to him.) We see that his natural personable nature and exquisite manners have not been excised by the lack of a finishing education in matters of court and etiquette and courtly behaviors. Oliver is most probably envious of the easy way his brother has with people,how people warm to him despite everything -in the way that people sometimes do - for no reason that we can quantify or measure - some people have a certan 'je ne sais quois' or 'charisma.' people gravitate towards warmth and generosity. Some critics think that Oliver shows him up in a bad light and wants rid of the comparison so he will have no rival.
The opening scene of Shakespeare's "As You Like It" reveals to us the sibling rivalry between Oliver the eldest son and Orlando the younger son of the nobleman the late Sir Rowland de Boys. Orlando is fuming with anger at the injustice meted out to him by his eldest brother Oliver. Oliver is the eldest son and so has inherited his father's title and all of his vast estate and his wealth, but he has forgotten his duties towards Orlando. He treats Orlando just like one of his servants and denies him the education that is rightfully due to him as a gentleman and a lord. In fact, Oliver has sent another brother Jaques to school to get a gentleman's education although he refuses to send Orlando to school.
The play begins with Orlando complaining passionately and heatedly to Adam their trusted and loyal servant and hoping that Adam would be able to supply a suitable explanation for Oliver's hatred towards him:
As I remember, Adam, it was upon this fashion
bequeathed me by will but poor a thousand crowns,
and, as thou sayest, charged my brother, on his
blessing, to breed me well: and there begins my
sadness. My brother Jaques he keeps at school, and
report speaks goldenly of his profit: for my part,
he keeps me rustically at home, or, to speak more
properly, stays me here at home unkept; for call you
that keeping for a gentleman of my birth, that
differs not from the stalling of an ox? His horses
are bred better; for, besides that they are fair
with their feeding, they are taught their manage,
and to that end riders dearly hired: but I, his
brother, gain nothing under him but growth; for the
which his animals on his dunghills are as much
bound to him as I. Besides this nothing that he so
plentifully gives me, the something that nature gave
me his countenance seems to take from me: he lets
me feed with his hinds, bars me the place of a
brother, and, as much as in him lies, mines my
gentility with my education. This is it, Adam, that
grieves me; and the spirit of my father, which I
think is within me, begins to mutiny against this
servitude: I will no longer endure it, though yet I
know no wise remedy how to avoid it.
Strange as it may seem, there is no explicit evidence at all in the play for Oliver's hatred towards Orlando his younger brother. Whatever reason we can think of - jealousy because he was liked and favored by the ordinary people, because he resembles his father more than Oliver - will only be a conjecture.
Oliver hate Orlando because Orlando had inherited his father's i.e,Rowland De Boys characteristics,this made Oliver jealous of him as ,the people of his own kingdom were sympathetic towards Orlando and completely ignored the good qualities in him.