The role of the Chorus is Oedipus is to act as the voice of reason. It's the chorus which prompts Oedipus to action (by documenting for us the curse which has apparently fallen on their town), asks him to be reasonable when he gets out of control (as in his confrontations with Tiresias and Creon), and holds him accountable when his past is revealed. The Chorus changes its opinion of Oedipus throughout the play, as do we; however, it is the reasonable, questioning entity through which we change our view of Oedipus.
Hamlet has a kind of chorus, as well, if we use the same definition: the voice of reason. Horatio is the voice of reason, of course, and Hamlet and others turn to him for a reasoned response to puzzling or perplexing things. When the palace guards have seen the Ghost but don't think anyone will believe them, they bring Horatio to see the spectacle. If he sees it and believes--for he is the consummate doubter of such things--they know they can move forward. Once he sees and believes, it is Horatio's voice which compels Hamlet to see for himself. When Hamlet is about to "catch the conscience of the king," he asks Horatio to watch with him; and afterwards Hamletseeks Horatio's affirmation of what he thinks he observed. As Hamlet is dying, he entrusts Horatio to tell the truth of what must have seemed to be inexplicable events to everyone but those who were no longer alive to tell the truth. Horatio is the arbiter of truth and reason, even at the end.