One thing Oliver says at the end of the opening scene is that he actually doesn't know why he "hates nothing more than" Orlando (I.i.166). Oliver further goes on to describe Orlando's virtues, such as the fact that despite Orlando never having had any formal education, he is very wise and knowledgeable; he is also noble and so "enchantingly beloved" by all that Oliver himself is hated by all, as we see in Oliver's lines:
Yet he's gentle, never schooled and yet learned, full of noble device, of all sorts of enchantingly beloved, and indeed so much in the heart of the world ... that I am altogether mispris'd. (166-71)
Literary critics have argued that Oliver's behavior is never fully explained, but we can surmise Oliver dislikes Orlando out of jealousy. He is jealous of Orlando's "talent, generosity, and aristocratic impulses" but does not see the same virtues in their youngest brother Jaques and so hates Orlando and treats him poorly but not Jaques (eNotes, "Oliver (Character Analysis)").
Oliver, from the starting of the play appears to hate Orlando. It is because though Oliver has done his absolute best in keeping Orlando away from good education, noble people, et cetera, he is still popular among the people of his kingdom. He also posseses noble qualities, which makes Oliver jealous. This is why, Oliver hates Orlando in the beginning of the play.