http://www.ogre3d.org/docs/api/1.9/
Since however there are still some people out there working with Ogre 1.8 or even Ogre 1.7, we also keep those available. That's how nice we are

The older versions can be found under the following link:
http://www.ogre3d.org/docs/api/X.Y (replace X.Y with the version number, for example 1.7).
Additionally, we create a generic link that will always point to the current stable version (currently 1.9):
http://www.ogre3d.org/docs/api/latest
If you should face any issues, please let us know.
BTW: Of course we also redirect the old "single point-of-truth" link to the generic one, so all old links should still work:
http://www.ogre3d.org/docs/api/html/index.html