How does Dickens present the character Oliver Twist as an innocent character in Oliver Twist?

Oliver Twist is innocent because he is incorruptible.

The subtitle of this book is “The Parish Boy’s Progress” for a reason.  A progress is a type of story where the main character is not influenced by the events that happen to him.  In other words, the character is completely static, rather than dynamic.  Dynamic characters grow from their experiences, while static ones do not.

Oliver is pure good.  There is no evil in him, and he is completely unaffected by evil.  He is naïve to the point of incredulity.  Dickens did this on purpose.  He wanted people to understand that children are born good, and only corrupted by influences out of their control.  Oliver represented all of the poor orphans whom society ignored.

Monks puts it very succinctly when he complains to Fagin that Oliver was not turned.

I tell you again, it was badly planned. Why not have kept him here among the rest, and made a sneaking, snivelling pickpocket of him at once?' (Ch. 26)

Monks, Oliver’s half-brother, wanted desperately for Fagin to turn Oliver to the dark side so that he could be disinherited.  It was useless though.  Although Fagin tried to teach Oliver to be a pickpocket, the boy was stubborn.  He spent his early days actually believing that Fagin’s boys made their own handkerchiefs and wallets!

That Oliver is good is evident in that he sees the good in others.  For instance, Oliver feels sorry for Nancy, a prostitute.  It is because Nancy recognizes that Oliver is not like the others that she risks her life to save him.  If Oliver had been corruptible, she never would have done that.

When Oliver is left behind during the robbery, it is his innocent looks that save him.

'But, can you—oh! can you really believe that this delicate boy has been the voluntary associate of the worst outcasts of society?' said Rose. (Ch. 30)

Oliver’s goodness saves him in the case of Brownlow.  Brownlow believes, despite the small likelihood, that Oliver is a good person.  He also takes a chance on Oliver.  In the end, Oliver proves him right.

If Dickens had a mission in life, it was to help suffering children.  His targets were workhouses, schools, and street gangs.  He was well aware of the dangers to poor children, and he constantly personified the social ills of England in kids like Tiny Tim, David Copperfield, and Oliver Twist.  Readers may be able to look away from beggars on the street, but they could not shield their hearts from these innocent waifs.