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 being done in parallel by Rockstar), the game's development was not completed 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. As a result, while the Mac betas had version number 1.0 as in Windows (see Windows read-me for version number), the retail version of the game app for Macs had the version number "1.1" (pictured, right; also see the Mac read-me). But even long after 1.1, the Mac Oni app would continue to evolve.
Classic to Carbon
As seasoned 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 still based on the System 1.0 code from 1984, and they were hoping to start fresh with a new OS as soon as possible. To that end, Apple acquired Steve Jobs' company NeXT to use the NeXTstep OS as the basis for their new OS. 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.