At vero eos et accusamus et iusto odio dignissimos ducimus qui blanditiis praesentium voluptatum deleniti atque corrupti quos dolores et quas molestias excepturi sint occaecati cupiditate non provident, similique sunt in culpa qui officia deserunt mollitia animi, id est laborum et dolorum fuga.
Et harum quidem rerum facilis est et expedita distinctio. Nam libero tempore, cum soluta nobis est eligendi optio cumque nihil impedit quo minus id quod maxime placeat facere possimus, omnis voluptas assumenda est, omnis dolor repellendus.
Itaque earum rerum hic tenetur a sapiente delectus, ut aut reiciendis voluptatibus maiores alias consequatur aut perferendis doloribus asperiores repellat.
That's a very large question. A few very sketchy answers:
(1) If the temperature is high enough, or the pH far enough from neutrality, then the Hb protein will denature, meaning it will unfold from its compact shape to form the shape of a loose coil.
(2) I don't think the number of O2 molecules bound to the Hb materially changes its overall shape -- that is, I don't think it's one of those proteins that undergo a significant conformational shift on ligan binding -- but I do know that the four separate domains of the Hb must "rock" apart in order for O2 to find its way to the central heme group. The channel in is closed in the normal conformation, and a certain kind of wiggle is necessary for it to open.
A lot of work has been done on the relationship between the conformational dynamics of Hb and O2 uptake, but this is pretty advanced stuff -- PhD in chemical physics kind of stuff, for the most part.