A good place to start if you are having to come up with a thesis on a novel is thinking through the themes that are presented through the action, plot and characters and then using one of those themes to create a thesis. There are certainly plenty of themes in this great novel by Dickens, and I have included a link to the themes section on enotes below.
However, one of my favourite themes from this novel, and, I think, the most apparent, is that of Good and Evil and how they are presented in the novel. Clearly the character who is presented as being most "good" is Oliver, and this is worthy of some analysis. He seems to remain completely incorruptible in spite of the range of hideously evil characters that surround him and try to pull him down. Yet a key criticism some people have is that this is at the cost of his development as a proper character. On the whole throughout the novel he is passive and is led easily by other characters.
Note too how other characters are shown as "evil" in spite of the social positions they hold that should make them "good." Mr. Beadle is a key example of this. The only character in the novel that seems to blend the opposing states of good and evil is poor Nancy, who appears to be unlike the rest in her desire to do good and look after Oliver, but at the same time recognising her "evil" background.
I think there should be ample material to write an essay on this theme. Perhaps a thesis you could use would be: "Characters in this novel are either presented as Good or Evil." This would allow you space to discuss Nancy and her presentation. Good luck!