When Oni was first released for Macs and Windows, the icon was this: . Mac icons at the time maxed out at 32x32 pixels. However, when Omni ported Oni to OS X, they used a 128x128 version of the 'O' icon instead of the original Konoko icon, so the 'O' became the new face of Oni for the Mac.
When Apple released 10.5 Leopard, icon sizes increased from 128x128 to 512x512 (then to 1024x1024 in 10.8 Mountain Lion). Although users rarely see icons at that maximum size, it does happen when a user sees a Preview in the Get Info window, or uses QuickLook on the application. When opening a file, the icon also undergoes a brief zoom effect. Here's an example of the zoom effect with an old 32-pixel icon and then with a newer, high-res icon:
Even though The Omni Group's 'O' icon was 128x128, it was still noticeably grungy when was zoomed in on. Here's a before-and-after:
I started tracing the icon using the vector illustration program Inkscape so it would be suitably high-quality for the Intel Mac build that Feral was working on. Here's a 512px comparison so you can see exactly how things changed:
Now that the Intel build from Feral is out, it seems appropriate to release the icns file, PNG renders, and the original SVG vector file for anyone who's interested.

ICNS
(click to download)

SVG
(right-click and save)

PNG
(right-click and save)
Large: 256x256

Gigantic: 512x512

Largest Intended Render Size: 1024x1024