It's on the entrance door ... the one to the furthest right.
OK, hard to see, I know. There's a better shot below. I just loved seeing this. And I really was lucky to see it - it was grafitti after all, and the next time I was there, the following week, it was gone, predictably. I mean, I agree with them, it ought to be removed. But if you're going to do something like this, then wow, way to go!! This is creativity in action, really. Brilliant. :-)
It says, in a translation that isn't word for word accurate, but is in fact more correct for it:
Knowledge is power, but ignorance is bliss.
Again, this was on the door to Oslo's main library. Gotta love it. :-)