What a fascinating question! These are two very interesting texts to compare and contrast. If I were you and I had to compare them, I would try to approach it from the vantage point of looking at the nature of evil and how evil is presented in these two texts. Clearly Macbeth is a masterful study of the downfall of a heroic and charismatic individual thanks to the intervention of witches and the whole concept of free will vs. destiny. Again and again the audience of this excellent play are caused to ask themselves whether Macbeth was right in acting to make destiny happen, or whether destiny will happen without any help from us. The tempting, seductive voices of the witches dominate the play as we question their ability to reveal the truth and whether Macbeth would have still gone on to do what he did were it not for their intervention.
The presentation of evil in Oliver Twist is very different. Here we are introduced to some characters, such as Fagan and Bill Sykes, who are inherently evil without any hope for redemption. In fact, it is notable that this text contains characters that are either "good" or "evil" with no in between. The one exception is Nancy, who seems to dwell between these two states, but perhaps when we understand her love for Bill and the pressures exerted on her we can say that she falls more into the "good" category. Oliver himself seems to be a study of the incorruptibility of youth. No matter what he experiences, he remains essentially good.
Hopefully these two paragraphs will give you some ideas of how to take your ideas further. Good luck! You have picked two interesting texts to write about!