As I'm writing this, "Anna Torv" is trending on Twitter and has been for a few hours (though there are sporadic patches when she isn't). That's saying something because when the series began, I remember loads of people attacking Torv, criticizing her wooden acting and lack of personality. It was a valid criticism, magnified by the fact that J.J. Abrams had earlier cast the ultimate kick-ass heroine in Jennifer Garner. But as the series progressed and Olivia's role expanded, Torv was able to show she could do more, which partially puts the burden on the writers to trust that she can handle the responsibilities.
Olivia doesn't really do that much in "Os," the most notable thing being her mood change due to being with Peter. It's the final twist everyone will remember, a near-perfect rendition of William Bell's coarse voice, because William Bell is now inside her body. As ridiculous as the soul magnet sounds, it actually works.
Plot-wise, the episode features a standard case about people floating. While it's not particularly exciting, we once again see the significance of the father-son relationship as well as the lingering effects of Walter crossing those years before. Apparently, now that the universes are unstable, the physical world no longer functions as it once was, and the two heaviest elements can combine to be lighter than air.
Score: 8.7/10
