History of Mac Oni
Windows 1.0 to Mac 1.1
While Oni was developed by Bungie West simultaneously for Windows and Mac OS (with the PS2 port coming at the end of development), the game did not cease development for both PC platforms at the same time. Oni was gold-mastered for Windows around mid-November 2000. The Mac version continued in development for at least another month. Most of that time was probably spent fixing Mac-specific bugs, but certain changes were made to the game as a whole (see PC vs. Mac for details; note that the Windows demo had its app built from this 1.1 code, thus it has the same differences from retail Windows Oni as Mac Oni does). This resulted in a game application for Macs with the version number "1.1". However, the Mac Oni app would continue to evolve over time.
Classic to Carbon
As long-time Mac users will recall, the Macintosh has been through three major technology transitions since 1984. First came the move from 68k to RISC-based PowerPC processors in the early '90s. Thus, when Oni began development in 1997, all Macs ran on PPC processors. Apple was about to release Mac OS 8, but it was based on code from 1984, and Apple was hoping to start fresh with a new OS as soon as possible. They acquired Steve Jobs' company NeXT to use the NeXTstep OS as the basis for their own. The Mac's second major transition, from OS 9 to OS X, started in 2000/2001, an awkward time for Oni.
In 2000, Bungie was merging into Microsoft, and Take-Two was assuming command of the Oni IP (as detailed in the Oni article). Having been in development years before OS X released, Oni had probably been getting built as a Classic app, referring to the Classic Mac OS (anything before 10.0). Upon its release, the best that Bungie could do for compatibility was build the Oni application as a Carbon app, meaning it was written primarily for Mac OS 9 but able to run on OS X. Unfortunately the Carbon build was not without issues in the OS X environment.
Carbon to Cocoa
Then, at the end of 2001, Omni Group released a Cocoa (native OS X) build of the game which they had produced for free. This became known as the Omni build (now usually referred to as the PPC build). Their new builds (released through 2003) kept Oni stable for several years. The last Omni build is still available here and here. However, as computers advanced, a critical bug cropped up on both Macs and Windows machines, where Oni's querying of the graphics card at startup caused a crash. Once the patch was figured out in Windows, it was carried over to the PPC build. Other patches were made in time; see here for details.
PPC to Intel
However, in 2006 Apple began their third major Mac transition: the switch to Intel processors, back to CISC architecture and moving away from the PowerPC chip for which both Bungie and Omni Group had built Oni. For a while, PPC apps could still be run in OS X using Rosetta, but support for the old architecture was expected to eventually phase out. In 2009, Feral Interactive (Oni's Mac distributor outside of North America) expressed an interest in putting out an Intel-native build of Oni. They obtained the source code from Omni Group, who had begun porting it to Intel, and made a beta release in 2011, shortly before OS X did away with PPC support. The release is stable, although it has a few non-crashing bugs, and incorporates a number of patches requested by fans, detailed HERE.