Oliver faces numerous challenges when he arrives in the big city. But here are just four of them:
Oliver doesn't know anyone in London. This would be hard for anyone, least of all a young orphan without friends or family. As soon as he arrives in town, Oliver needs to find his way about, and that's easier said than done.
The second challenge is related to the first. As Oliver doesn't know anyone in London, he's vulnerable to exploitation. And indeed, that's precisely what happens to him. As soon as The Artful Dodger lays eyes on the poor, half-starved waif, he knows that Oliver would make a great addition to Fagin's gang of thieving young vagabonds. So he takes him back to the den, where he will be used by Fagin and Bill Sikes to commit thefts and participate in burglaries.
On arriving in London, Oliver is very hungry after his long journey (on foot!). At least back in the workhouse he had something to eat, even if it was just a bowl of slop. But here in the bustling metropolis, he has to figure out a way to get food for himself, and that's not something he's ever had to do before.
Finally, Oliver needs a place to stay. In early nineteenth-century England, especially in big cities like London, homeless youngsters on the street were a depressingly common sight. There were no government social or welfare agencies to help children in distress, so young people like Oliver, without any money or family, were forced to find shelter wherever they could.