Oliver's innocent in that he's never known his true identity. He's grown up an orphan in considerable poverty, without any proper guidance from adult authority. The workhouse, despite the Bible verses adoring its grim walls, doesn't provide anything in terms of a moral code for Oliver to follow. It's a place of hopelessness and desperation in which poor young waifs are overworked, underfed, and cruelly beaten.
Yet despite all the terrible experiences of his formative years—in the workhouse and elsewhere—Oliver doesn't develop into some kind of juvenile delinquent. Even though he falls in with a gang of professional criminals—made up of juvenile delinquents—he still manages to retain a sense of innocence. It's notable that Oliver is reluctant to accompany Bill Sikes on the Chertsey burglary, and only goes along out of fear of what this vicious brute might do to him.
Amidst all the filth, the squalor, the crime and moral corruption, Oliver's innocence somehow shines through....
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