Oswald, Goneril's servant, is your typical slime-ball. He does whatever Goneril wants him to do, immoral or not. Oswald's fight with Kent is symbolic: a loyal servant who has his master's best interests in mind (Kent) battles it out with Oswald, who's just a self-interested sycophant (person who flatters insincerely to get what he wants).
In Oswald's defense, however, he is loyal in his own way—he refuses to betray Goneril. Oswald keeps his mistress' letter to himself because, it seems, there's some honor among evil and treacherous thieves. Oswald makes us do what we really don't want to do, that is, look at the evil sisters as real people.