The coincidences in Oliver Twist are pretty extraordinary, to say the least. In the hands of a lesser writer they'd be ridiculous and absurd. To take but one example, Oliver is forced to go with Bill Sikes to burgle a house one night. The burglary goes badly, and yet the family whose house was broken into take pity on Oliver and care for him. The owner of the house, a certain Mrs. Maylie, has an adopted daughter called Rose. In one of the many astonishing coincidences in the book, Rose just happens to be Oliver's aunt, the orphaned sister of his mother. Rose is instrumental in persuading Nancy to help save Oliver from the evil clutches of Bill Sikes, Monks, and Fagin.
It's arguable that this particular coincidence is therefore very important for the subsequent development of the plot. Furthermore, the chance meeting of Oliver and Rose is entirely in keeping with the overall tone of the book. Oliver and his aunt are held together by a bond of love and affection before they become aware that they're actually related. As is often the case with Dickens, high sentimentality is never far away. But as with the accompanying coincidences, it's a sign of Dickens's talent as a writer that his rich characterizations and remarkable eye for detail provide more than adequate compensation.