After leaving the village, Oliver is exposed to criminals and a life of crime but he remains honest and good.

Oliver is sent from the workhouse after asking for more gruel.  He is given to an undertaker, but his lowly condition leads to conflict with his employer and the other servants.  Oliver decides to run away.  He explains this to an old friend before leaving.

I am running away. They beat and ill-use me, Dick; and I am going to seek my fortune, some long way off. I don't know where. (ch 7)

Oliver clearly is not willing to just lie down and take the abuse.  He fights back.  He runs away.  Oliver is ready to make his life better.

As Oliver progresses through his life, he does not really change.  He continues to be innocent and uncorrupted.  Things happen to him.  At this point he runs into Dodger, who tells him he can take him to a kind gentleman who will give him a place to sleep.  Oliver goes, not realizing that Fagin is a criminal.

In Fagin’s lair, Oliver is exposed to the criminal life—but he doesn’t realize it at first.  He is told that Fagin and the boys are just playing a game.

[The] merry old gentlman and the two boys played at a very curious and uncommon game, which was performed in this way. The merry old gentleman, placing a snuff-box in one pocket of his trousers…and trotted up and down the room with a stick… (ch 9)

Oliver learns to play the game, and helps to get merchandise ready for sale without realizing what he is doing.  It is not until he and Dodger go out to pickpocket that Oliver discovers what they are doing, and he is not willing to become a thief.