In this play, Oliver and Orlando are sons of the late Sir Rowland de Boys. Oliver is the oldest son and Orlando is the youngest. There is one other son that we know of.
In those days in Europe, all the property and the title of noble European families was passed to the oldest son and the other sons got nothing. The oldest son was in control. This is what caused the conflict in this case.
Oliver, for some reason, does not like Orlando. There is a lot of speculation about why, but no proof in the text. At any rate, he has treated Orlando poorly and Orlando is unhappy about this. So they both dislike each other.
We do have evidence why Oliver hates his youngest brother Orlando. From the very beginning he is very jealous of the noble qualities of his brother that people praise. So he wants to be the one to be praised and is so cruel and selfish that he deprives him of all education, inspite of his father writing in his will that Orlando should be brought up with a noble upbringing. Still Orlando grows up to be a perfect gentleman like his father.
When Orlando comes to know of the will by his faithful servant Adam, he is angered and says that his tolerance has vanished, he can no longer bear this tyranny. On the other hand Oliver decides to put an end to Orlando's demands once and for all by killing him. This is the cause of the conflict. He thinks of various ways to do this.
Later on he employs the strongest wrestler in the town, Charles, to kill his brother. But Orlando defeats him too. (It is here that he meets Rosalind). This angers Oliver all the more and plans to set fire to Orlando's lodge while he's asleep. Thus the conflict continues.
the simple cause is that oliver treats Orlando very badly..
& still Orlando who has all the good qualities of his father (even when he is not schooled & well bread)& does not oppose his brother till he is young & mature enough...
& so all the people who know oliver & orlando think that oliver is evil & support orlando, which in turn makes oliver even more angry, ultimately resulting in him treating Orlando badly & not giving the thousand crowns to Orlando (left by Sir Roland de boys for Orlando) ..
this treatment of oliver makes oliver angry & as a result even Orlando hates oliver...