It is hard to understand why Oliver treats his brother with such disdain. One reason might be that Orlando is so much stronger. Orlando is also probably better looking, since both Rosalind and Celia are smitten by him at first sight. Shakespeare often uses common human traits in his plays with some slight exaggeration. In the case of Oliver and Orlando, Shakespeare is using sibling rivalry to advance the plot. Both men are proud, but Orlando is honest and honorable, whereas Oliver is treacherous and jealous.
Orlando is justly resentful because Oliver, his older brother, refuses to help him improve his education but treats him like a peasant. He says:
My father charged you in his will to give me good education: you have trained me like a peasant, obscuring and hiding from me all gentlemanlike qualities.
Orlando actually encourages the formidable professional wrestler Charles to kill Orlando in their upcoming bout, but Orlando wins and then is forced to flee because he learns that Oliver has further plans to kill him. Duke Frederick orders Oliver to pursue Orlando because he believes Rosalind and Celia have run off with him and he blames this on Oliver.
The Duke's reason for blaming Oliver needs explaining. It is that he suspects him of plotting to kill his youngest brother and forcing him to flee, thereby providing Rosalind and Celia with the protection to flee with him. In other words, the Duke believes that his daughter Celia is to be found with Rosalind and that Rosalind is to be found with Orlando because they are in love with each other. This is not exactly correct, although Orlando and Rosalind do end up meeting in the Forest of Arden.
At the end of the play the brothers are reunited and reconciled in the Forest of Arden, where Orlando saves Oliver from being killed by a lioness. Here Orlando behaves with characteristic strength and courage. Oliver has become a changed man. He promises Orlando that he will give him his estate and all its revenues (V.2) because he wants to marry Celia and live the simple life of a shepherd in the forest.