And I'll second the bit about TAing, though my experience was in physics rather than CS. Being a physics lab TA is cool...you get to play with all sorts of stuff. (Heck, taking physics labs is cool, though the lab reports are not so cool.)
But OS... if you don't know that stuff you're basically worthless.
Actually, I'd probably recommend it more for someone planning to go into finance, economics or history (!!) rather than someone going into physics or CS, unless you're going into a subfield directly related to stat mech. Most of my physics coursework has been basically useless for my computer programming; yeah, it's nifty to know your vectors and rotation matrices (particularly if you go into graphics), but I've never had to solve a volume integral or contract a tensor since dropping my physics major.
Organic Chem - 1 credit.
OS - 1.5
How many credit hours is considered a full time load at your schools?
So the CMU OS course is equivalent to four normal classes.
BUT... but I just looked up the class at CMU and it is not 12 credits. It is 4 credits. Which shakes my worldview somewhat, I thought this 12 credit was proof that the CMU CS curriculum was something special.
So either I was mislead by my friend at CMU years ago when this course came up in discussion or more likely, I misunderstood him. Feel free to downmod my post above, it would ideally settle out around 0 points for containing wrong information. Sorry for spreading untruths...
And if you have a chance to TA, do it. You learn a lot more by teaching than you do by being taught.