Mr Charrington and O'Brien are similar as they first appear to be on Winston's side and against the Party but then end up betraying him. They disguise their real intentions from him; in fact Charrington uses a literal disguise of being a benevolent old man when really he is a young, ruthlessly efficient member of the Thought Police. Similarly O'Brien, a senior member of the Party, initially pretends to help Winston's rebellious activities. He tricks Winston into believing that he is part of an inner Party revolt, when really he is loyal to the Party all along, and eventually oversees Winston's torture, interrogation, and manipulation. However, he is not a one-dimensional villain; although completely in service to the Party and its utterly remorseless pursuit of pure power, he is actually quite intelligent and understanding.
Both Mr. Charrington and O'Brien are members of the Party, both are intelligent and knowledgeable, and both deceive Winston and Julia.
Both men present as benign. Mr. Charrington, though in reality much younger, presents himself as a harmless and humble 63-year-old shopkeeper in a velvet jacket. He pretends to be a prole with a Cockney accent. O'Brien has a blunt, pleasant look in his black overalls and pretends to be part of the underground Winston is seeking.
Both men supply Winston, insatiably searching for knowledge, with information he craves. Mr. Charrington recites to Winston part of the "oranges and lemons" rhyme and even explains some of the history and the background of the verses. O'Brien invites Winston and Julia to his apartment, offers them wine and give Winston "The Book," supposedly written by Goldstein, that explains oligarchic collectivism. Both men thus win Winston's (and Julia's) trust while betraying them.
Both Mr. Charrington, a member of the Thought Police, and O'Brien are ruthless men dedicated to the Party. They know how to manipulate others. Both are frightening because they are able to fake being humane and benevolent so successfully. What is most troubling about both men is that they clearly have the knowledge, personality and intelligence to make other choices and yet have chosen a dehumanizing path.