The main difference between the 1968 movie "Oliver!" and Charles Dickens' novel "Oliver Twist" is the overall tone of both works; the movie also skips over much of Oliver's early life. The film is a musical, with light-hearted and uplifting moments that are largely absent from the bleak atmosphere expressed in the book.
Despite the tonal differences and songs, the 1968 film manages to remain relatively faithful to the main plot elements and characters in the book. Dickens had intended his novel to bring attention to the plight of England's citizens living in abject poverty, and much of his book depicts the harsh real-life conditions encountered by the titular orphan. While the film retains some of this feeling (especially in a famous scene where young Twist asks for a second helping of gruel), ultimately the singing and well-choreographed set pieces provide a lighter, more happy atmosphere.
The original book also goes into great detail regarding the squalid conditions typically experienced by impoverished citizens living in London in Dickens' time. These background chapters culminate in Oliver's birth and abandonment to a life in poverty, and detail how he ended up at the large workhouse where much of the rest of the novel is set. The movie commences with Twist and his compatriots already toiling in the public workhouse, serving a lavish banquet for upper-class citizens.