Yay, I finally got standalone Qt4 to acknowledge the presence of KDE's Oxygen style. I had to symlink

ln -s /opt/local/lib/kde4/plugins/styles /opt/local/share/qt4/plugins/

And then you get this (cropped to comply with forum requirements) ...

Hmm, that is unfortunate. Are any messages printed to the terminal which might indicate why it appearing so visually distorted?

No, nothing. Whatever happened is because of a design issue that doesn't lead to execution errors.

Damn. I'm guessing that all debug areas are enabled in kdebugdialog?

Ok, I'm flabbergasted.

KDE's oxygen-demo app worked perfectly all of a sudden, as seem to do the other ones I tried - using the oxygen style of course. Something I did yesterday (or over the course of the last week trying to get Qt to propose the style) solved the rendering issue under KDE. Unless it was the recent update pushed to the kde4libs, or the fact I'm now starting kded4 manually. Good news in itself, but the annoying fact is that now I cannot get back the rendering issue to come back. Who knows after a reboot ...

Qt continues to misrender the Oxygen style, but frankly that suits me. AFAIK Oxygen is a KDE style, and I'd actually prefer to let unrelated Qt apps blend in better with OS X.

One possibility is that certain applications shipped with Qt itself are hardcoded to a certain extent to use certain styles depending on the platform - and the ugly appearance is caused by the two styles conflicting.

Please post back here if the issue resurfaces in normal Qt applications (those not shipped as part of Qt itself) or KDE applications.