I think that you could also look at the use of disguise and misrepresentation in the novel. The most obvious is Oliver, the "twist" who is mistaken for rags when he actually comes from riches. The Monk's perpetuate this mistaken identity in order to serve their own aims of claiming Oliver's inheritance for themselves. Nancy pretends to be Oliver's sister by putting on middle class clothes in order to get Oliver back to Fagin. Clothes as a means of identifying social class are important. Oliver is stripped of his clothes and pout into pauper clothes. If I were writing a thesis for a paper on this work, I would consider looking at the outward representations of class and status as displayed by clothes and manner of speaking, and the way that these "disguises" can and are used to meet individual desires.

Dickens, in fact, uses this theme as a means of satirizing class structure in Victorian England. You could even gop on to look at other class representations that are symbolized by dress, such as the way in which the Fagin boys almost worship handkerchiefs which they like to steal. These are symbols of affluence. If you carry a handkerchief, you are assumed to be a member of the upper class. If you wear the clothes of a workhouse person, you are seen as a pauper. Again, this is a way Dickens uses outside appearances as a means of denoting class differences from the perspective that people are viewed by society and judged by society by the clothes that they wear.

What would be a good thesis statement for the book Oliver Twist?

If you have read any of Dickens's other works or researched this author, you may, then, be familiar with a prevailing social concern of his:  the dehumanizing effects of crime.  Certainly these effects are evinced in the characters of Oliver Twist.  Now, if you wish to compose a thesis on this topic, you may wish to limit your examination to one character if your assignment is an essay.  However, if your assignment is a paper, then you will want to analyze several characters.

Oliver Twist is a narrative in which the most depraved villains preside, with the heinous Fagin controlling the others.  That he loses all humanity is apparent throughout the novel, most strikingly at the end when, incarcerated and alone in his depravity, Fagin loses his mind. As a metaphor of this transformation, the turnkey asks him,

"Fagin, Fagin! Are you a man?"

"I shan't be one long," he replied, looking up with a face retaining no human expression but rage and terror....

Another character, the murderous bully Sikes, evolves throughout the novel into an absolute monster.  Under his control, poor Nancy struggles to retain her humanity, regaining it in her solicitous acts on behalf of Oliver, but falling victim to the band of criminals who will not release her.  In contrast to Oliver, his half brother degenerates spiritually until his final end. For a time, Oliver even suffers some dehumanization and he worries that he will never be believed or treated fairly even if he escapes the evil band of thieves who have captured him. 

What would be a good thesis statement for the book Oliver Twist?

I assume from your question that you yourself need to design an essay question based on this novel and need to construct a thesis statement to answer it. Whenever you are given an assignment like this it is very important to start off by focusing either on a character or a theme that is present in the novel and then using that to construct a thesis statement. One element of the novel that is very clear is the way in which Dickens presents us with characters that, with one exception, fall very neatly into one of two categories: good or evil. Of course, it is Nancy who is the exception. Oliver in particular is an excellent example of a character who remains uncorrupted in spite of the company he is forced to keep and the activities he is obliged to engage in. Therefore, you might like to use these ideas as the basis for your essay. A good thesis statement would be:

Oliver Twist is a character that maintains his goodness in spite of his circumstances and situation in life.

This would allow you to examine his character, which is of course a flat character, because he does not really develop or change, in the context of the novel and what happens to him.