The case portion will be overweighted: you can move to 2nd round if you were just ok on the fit but amazin in the case; you'd go home if the reverse were true.
Nothing really specific about Oliver Wyman, you prep should be similar to other consultancies. Ultimately, every company really looks for the same perfect candidate anyway. Just be sure to review the mechanics of the case, knowing it will be slightly easier to move to a 2nd round than it would be at MBB. Cases are probably just as hard, with perhaps a higher likelyhood to get a market sizing portion.
As for the fit, standard questions: tell me about yourself, describe a time you face adversity, what are you proud of, how would colleagues describe you... The usual :)