Operating Systems is typically taught by Jason Nieh. This past semester it was taught by Prof. Nahum.
Overall, Nahum is an interesting lecturer who tries to engage the class as often as possible during lectures. He is very approachable and very helpful as well, if you are in a bind (say if the pressures of the world make you and the rest of the class need a hw extension).
Going by the number of assignments (and what Prof. Nahum said at the beginning of the semester) the class is a bit less hardcore than when Nieh last taught it, but all the important bits are still intact and you still learn a lot.
If Nahum teaches OS again I would definitely recommend it, as he is a pretty solid teacher.