What are the main themes in Oliver Twist?

There are a number of themes in Oliver Twist but here are two of the most important:
The evils of the workhouse system—Workhouses were established in early nineteenth century Britain as a means of dealing with the widespread problem of vagrancy. Instead of being allowed to beg and receive assistance "out of doors" as it was called, those who were absolutely destitute were to be herded together into workhouses. Here, families would often be separated and forced to toil for long hours doing menial work and receiving inadequate food.
Dickens was a trenchant critic of the 1834 Poor Law which brought the workhouse system into effect. Like many others, he believed that it criminalized the poor. He correctly surmised that workhouses were simply a money-saving device. They existed not to help the poor but to make life so utterly wretched for them that they would be deterred from seeking poor relief.
How adults exploit children—It is no fun being a child in a Dickens novel. His own deeply traumatic experience in a blacking factory as a boy influenced his sympathetic portrayal of children in Oliver Twist and elsewhere. Oliver is not just a victim of a savage, coldhearted world in which the poor are treated abominably; he is also the victim of the stern, unbending attitudes towards children widely shared by adults at the time. Adults do not simply exploit children; they feel themselves entirely justified in doing so.
In his withering indictment of pre-Victorian society, Dickens tellingly makes no distinction whatsoever between adults in positions of lawful authority and common criminals. Mr. Bumble is a humbug and a hypocrite who beats Oliver at the drop of a hat and ultimately conspires with the malevolent Monks to cheat him out of his inheritance. The supposedly respectable townsfolk in charge of the workhouse dole out glib homilies along with the bowls of indigestible slop they use to feed the malnourished children in their care. The odious Noah Claypole sneers at Oliver, taunting him about his mother's death before himself becoming a pickpocket. The list of abusive or exploitive adults is seemingly endless.
The criminal classes do not treat children any better, but at least they provide Oliver with better food and lodgings than ever receives in the workhouse or even at Mr. Sowerberry's funeral parlor. Fagin sees Oliver as ripe for exploitation. He is just the right size for picking pockets without being caught. His size is also useful for the violent psychopath Bill Sikes, who can take Oliver with him when he goes burgling as he fits perfectly through a small window.