Dear A,
The interview at the Oliver Wyman usually consists of 5-6 separate interviews.
1st - with HR, about your personality and your motivation for consulting and Oliver Wyman.
From the 2nd - mixed interviews, where you have personal fit and case part. At the begining FIT part is bigger than the case, but towards the end you would have almost entire interviews consisting of cases. But it's important to practice both - personal FIT and cases.
You also need to be very strong in your quantitative skills as you would definitely have many cases from the financial services.
If you need any further tips, I have already helped several candidates to get offers from Oliver Wyman, so feel free to drop me a line and I will further insights with you to help you get your offer.
Best,
André