I would argue that the Oliver Wyman numerical and verbal reasoning tests are some of the hardest out there - both in terms of difficulty and time pressure (based on my experience from the recruitment process for the London office).
It's also worth noting that the style of the OW tests differs significantly compared to other companies (particularly the numerical reasoning test). The best examples I've seen online of typical questions can be seen here:
Several questions very similar to those shown in the link above came up during my assessment, so do make sure to complete and understand each question as part of your preparation - just in case!
Hope that helps and good luck.