If you're talking about MBB, then only McKinsey lets you specialize immediately post-MBA through their different practices, like operations or business technology. Some smaller firms like LEK or A.T. Kearney are well known for operations expertise, though that may not be all that they do. From what I've heard as a new consultant you'll have little control over what kind of project you're selected for so I wouldn't necessarily count on being able to do ops projects as a generalist.