As the other educators have made clear, Oliver Twist is a novel and poverty is one of its major themes. Specifically, Dickens deals with the terrible effects of poverty on an individual's life and nowhere is this more apparent than in Chapter Two when Oliver famously asks for more supper. Described as "desperate with hunger" and "reckless with misery," Oliver endures the type of poverty and deprivation which inspired Dickens to write the novel in the first place and to which he hoped to draw the public's attention.

Moreover, this theme is also explored in the reaction of the workhouse staff to Oliver's request for more supper. The master hits Oliver with the ladle, for instance, and the beadle is stupefied that Olive would ask for more after eating his "allotted" portion. By portraying the workhouse officials in this way, Dickens employs satire to great effect. He shows us that his contemporaries treated the poor in a way which dehumanised and deprived them instead of actually helping them to improve their lives.

In Charles Dickens full-length novel Oliver Twist the major theme is the classic theme of Good versus Evil. Dickens said that he created Oliver to represent the principle of good surviving through manifold adversity and "triumphing at last." The novel has characters who are completely bad like Fagin and Bill Sikes. It also has characters who are completely good like Oliver and Rosa. It also has characters who may have good characteristics but are in bad circumstances that precipitate bad behavior like Nancy and Artful Dodger.

There are also characters who represent society's ideal of goodness because of their trustworthy or high position who are in fact corrupt like Mrs. Mann, Mr. Bumble, and Monks' mother. Through these representations of good and evil and shades thereof, Dickens explores good triumphing over evil.

A novella is a work of fiction that is in between the lengths of a short story and a novel. Edgar Allen Poe once said a short story is something you can read in about an hour. A novel takes many hours to read, especially one as densely constructed as Oliver Twist. A novella lies in between. A somewhat arbitrary definition states that a novella is between 20,000 and 50,000 words. The same definition states that the novel genre begins at 50,000 words.

The primary theme in "Oliver Twist" is the age-old battle between good and evil.  As quoted in Enotes, Dickens once wrote that he "wished to show, in little Oliver, the principle of good surviving through every adverse circumstance, and triumphing at last".  To this end, the characters in the story tend to be rather single-dimensional, with a few exceptions, they are either very good or very bad.  Characters like Oliver, Mr. Brownlow, and the Maylies are virtuous, and those like Fagin, Monks, and Sikes are evil. 

A second theme in the book is the effect of British laws during the first half of the 1800s on the poor.  Reasoning that the enactment of "Poor Laws" would save much money on the care of the indigent, the government confined those mired in poverty in workhouses, where they were starved and mistreated with no avenue of redemption.  These laws paradoxically resulted in the development of a seedy underground of crime, and rates of lawlessness soared.

Alienation is another theme that the story explores in depth.  Each of the characters are alienated both from each other and society.  As an orphan, Oliver is the stereotypical outcast, and, with the possible exception of Dick, the people in his life are only out for themselves.  Although the good people in the story do share moments of community and trust, their numbers are small, and they are far outnumbered by their unscrupulous counterparts.

For a much more comprehensive discussion on themes, check out the second Enotes link below.  Good luck!