The most obvious portrayal of the themes of injustice and usurpation is with respect to Duke Frederick's treatment of Duke Senior. Duke Senior is the eldest brother, so by rights, the dukedom belongs to him. However, Frederick became jealous of Duke Senior's power and usurped him, driving him and his courtiers into exile in the Forest of Arden. Naturally Frederick's usurpation of Duke Senior was also an injustice.
We further see the theme of injustice and usurpation portrayed in Oliver's treatment of Orlando, although this is a slightly different case. Their father Sir Rowland has died, leaving Oliver as the eldest brother the title as well as the estate. However, Sir Rowland left provision in his will for both of his younger sons. He granted them both one thousand crowns plus made Oliver promise to see to their education as gentlemen. Oliver has given their middle brother Jaques his inheritance as well as seen to his education, but he has denied Orlando, their youngest brother. To deny or seize from someone what legally belongs to that person is a form of usurpation. Hence, even though Oliver has not taken a title and estate away from Orlando, just as Frederick took away Duke Senior's title and estate, Oliver has still usurped Orlando's legal rights. What's more, Just like Frederick was jealous of Duke Senior, it is out of jealousy that Oliver has denied Orlando his fortune as well as his education, and Orlando is feeling the injustice of such treatment. We particularly see Orlando speaking of the injustice of the treatment when he complains to their longstanding family's servant Adam that Oliver is treating the estate animals far better than he is treating Orlando, as we see in Orlando's lines:
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. (I.i.10-14)
Hence we see that both usurpation and injustice are very dominant themes in the play.