With Ubuntu 11.10 Oneiric Ocelot, there is no classic Gnome 2 desktop and there is no way to install it, as Ubuntu 11.10 uses Gnome 3. However, “fallback Gnome” can be installed which is very similar to the classic Gnome 2 desktop.
Open the terminal and run:
sudo apt-get install gnome-session-fallback
After the installation is complete, log out or restart the computer.
At the login screen, select the gear icon and choose one of the Gnome classics.
Log in as normal.