Hi,
Agree with Nuno!
- You can have some "strange" fit questions, like "What you will do if you become a president". Seems you can't prepare for these
- Most probably you will have several brainteasers, so read the books "How to ace brain teasers interview" and "Are you smart to work at google?"
- In addition to regular cases prepare for market sizing since this is the most common type in unstructured interviews
- Since Oliver Wyman is heavily specialized on banking - learn a lot about it. (i.e. corporate and retail banking products, how bank accounts profit = interest income + commission income + transaction income - funding costs - provisions - FC, etc)