With a strange move for an open source project, KDE 4 is quite secretive about the new icon theme. The plan is to present the whole suite together, keeping Oxygen as a "novelty", so, even if Oxygen is being currently developed and most icons are already done, there are no official downloads provided for Oxygen. Moreover, the KDE 4 developers somehow "discourage" using Oxygen on a KDE 3 system.
However, Oxygen is open content (Creative Commons Attributions-Share Alike and LGPLv3 dual-licensed) and it is online in the KDE 4 SVN repository. So nothing legally prevents you to download and try the current Oxygen development version on your system.
It is not straightforward to obtain a KDE 3 -installable theme from the KDE 4 SVN repository, however someone (as often happens in the OSS world) has done the dirty work for you. After a bit of googling I found a handy script that automatically downloads the latest revision of Oxygen icons, converts them (using Inkscape for that purpose, so check you have it installed) and packages it as a theme (that is a single .tar.bz2 file) in your home directory.
The script is a bit slow, but works: basically just download the file from the webpage above, unzip it, open a shell, enter in the directory, run ./install.sh as a normal user and wait. When it has finished, just look for the Control Center in your KDE menu and go through Appearance and Themes --> Icons --> Install new theme and open the oxygen-icon-theme.tar.bz2 file in your home directory created by the script. Voilà, theme installed.
How does it looks? Really professional and cool. I must admit however that it can look somehow out-of-place on a common KDE 3 system, so probably KDE 4 developers are right -to fully experience Oxygen, better wait KDE 4 at the end of the year. However, here is some screenshot for your pleasure: 1 , 2 , 3 , 4