I'm not sure what your actual question is regarding this theme, but oppression and cruelty are certainly things we see pop up in Beckett's play. Arguably, Lucky and Pozzo are the two characters who are connected to these concepts the most, since Lucky basically functions as Pozzo's slave and is certainly treated cruelly in his oppressed lifestyle. However, it is also posited by the characters that this is potentially a choice for Lucky, thereby giving some sort of purpose or meaning to his life, however unfortunate said purpose may be. This ties in with the existential elements of the play which tell us that all humankind is struggling with a search for meaning in a meaningless universe. So, while Lucky and Pozzo have this, they are not any better off than Vladimir and Estragon, who seem to have no direct purpose involving each other, no required roles to play. Granted, they are waiting for Godot, but they do so in a tragicomedic way.
The theme of oppression and cruelty in Waiting for Godot has all sorts of metaphoric dimensions--Existentialist, theological, comic and even socio-political. As Hamm summed it up at the beginning of Endgame, we all think ourselves to be the greatest of sufferers and it is this self-assumption of a tragic status that turns it all into comic. Human existence, as the absurdist critic of the play would say, is full of oppression and cruelty and all of it stems from absolute chaos, lack of moral justice in a world that is random and absurd. More precisely put, as Camus would say, the link between the man and the world is absurd. The suffering of man is not always the lofty ones, Estragon's pain while trying to put on his boots is an example per excellence of human suffering and the irony is one blames it on the size of the boot.
The idea of oppression and cruelty recurs through the play in tandem with the christian myth of Damnation and Fall. The Augustinian reference to the thieves and the Fall of the four characters in the second act are all replete with theological metaphor. As vladimir's speech about his responsibility to the reigning chaos reveals, the thematic structures of oppression and cruelty are also resonant with the historical angles of the World War. However, there is always the shiftiness of power as we see between Lucky and Pozzo. The oppressive Pozzo suffers in the second act and has to depend on Lucky to show the way 'on'. The humane unity of Didi and Gogo holds on as opposed to the themes of oppression and cruelty.