The people at Oliver Wyman are very laid back and down to earth. I had 3 interviews in the first round (2 case interviews and 1 CV interview) and one math test. The math test was really hard since you only had about 30 seconds for each question and it took at least 15 seconds just to understand many of the questions. I only scored about 50 % but it was enough. In the second round I had 3 case interviews. One of the case interviews was a brain teaser where the interviewer was more interested in how I structured my thinking than the actual answer itself. The brain teaser was really hard. I later tested it on 4-5 other students. Nobody came up with the right answer.