I am an undergraduate looking for a career in strategic consultancy. I understand that the big players are McKinsey, BCG and Bain. How does Mercer Management Consulting compare? Is it on a par with OC&C, PA, Roland Berger..?

For grad recruiting, you wouldn't typically expect to see PA in the same list as those others (but you would expect to see Monitor and LEK, for example). Similarly, candidates wouldn't normally have Mercers in the same list except for their financial/risk operation, Mercer Oliver Wyman - which would be included, amongst other reasons, for their reputation for offering an astronomical starting package. They don't get turned down very often....

Mercer Oliver Wyman, little is mentioned of them on this forum, how comes? Astronomical strating packages...such as?

I'd guess because they are so specialised in the work they do. Package for new grads tends to be 5-10k above the rest of the field, which usually gives them the pick of who they want....