On a technical level, OS/2 is not related to DOS, UNIX, or or Windows. Though it is often mistaken as a DOS or Windows based OS and even UNIX. It only shares some design similarities with these; and can run apps many apps made for them. It is able to run programs from other OSes through compatibility layers and virtualisation (IBM called them "extenders"). For a short while, OS/2 was more compatible with DOS than Windows was. Originally the DOS compatibility required OS/2 to surrender complete control of the system to the single DOS program that was running (similar to Windows' "MS-DOS Mode" but without a reboot). 2.0 and later editions used a virtual mode to allow multiple DOS apps to run as virtual machine clients. Xenix and other UNIX apps could be run the same way. There was no actual 'Windows extender' for OS/2. It derives its Windows 3.x compatibility through a DOS extender capable of running modified versions of Windows 3.x (OS/2 version 1.0 contained Windows 3.0, OS/2 2.0 required Windows to be purchased separately.) Later editions improved the virtual machines enough to run 32-bit Windows applications without buying Windows. This was done in user-mode rather than at a hardware level. As a consequence, device drivers for Windows were useless for OS/2 systems. OS/2 executables use the .exe extension.
OS/2 also has some officially licensed Amiga technology under the hood. Specifically, it is compatible with the REXX scripting language that was also added to the last editions of PC-DOS. It is speculated that Amiga derived code is what keeps OS/2 from being made opensource.
OS/2 Warp 4 added Java support.
IBM attempted to expand its user base by distributing a demo CD-ROM of OS/2 Warp in the UK. An easy to crack back door was included to allow users to gain full access to a full version of OS/2. Thus bypassing agreements with Microsoft that required licensing fees and prohibited free distribution of their competing product that happened to be fully compatible with Windows applications. Australian Personal Computer Magazine gave away pre-cracked copies of the OS/2 demo without so much as a 'please no' from IBM.
A PowerPC version of OS/2 was released also. Several OS/2 games with entries in UVL are also PowerPC compatible and their's even the rare PowerPC only game. They are tagged with ppc and x86 if applicable. But OS/2-PPC was even more obscure then the PowerPC version of Windows that Microsoft released (Windows natively on an New world Macs; how bazaar). Rounding out the bazaarities, OS/2-PPC included OpenGL and similar DOS compatibility as OS/2-x86 using virtualisation, thus the same Windows compatibility. Similar, but not the same; OS/2-x86 DOS was based on PC-DOS 5 but OS/2-PPC DOS was based on PC-DOS 7 with added REXX support. For some reason IBM had not bothered to use these efforts to update OS/2-x86 DOS to version 7 at the same time. OS/2-PPC didn't run on Macs, only IBM Personal Power Series 830/850, Thinkpad 820/850, or IBM RS/6000 series. The core of the OS was the IBM micro kernel, based on the MACH kernel; having no similarity to the OS/2-x86 kernel nor any other operating system at the time (micro kernels have never seen widespread use in real world applications). It is rumored that anyone buying a 32-bit PowerPC machine from IBM can receive OS/2-PPC, with development tools, for no additional cost, simply by requesting it. Which is really of no use unless one wants to develop a proprietary application no one else will ever use.
OS/2 support by IBM was officially discontinued on 2006-12-31. However, Serenity Systems, with IBM's full cooperation, continues to offer support for OS/2 and their own improved version called eComStation. Also, some fairly big customers still use OS/2 and IBM offers fee based support on a case by case basis. At the same time, they highly encourage these these customers, and offer help, to migrate to Java as much as possible as a path to an eventual migration to Linux.
eComStation is essentially an OEM version of OS/2 with many enhancements. eComStation gaming, OpenGL, Mesa3D, 64-bit, is not a priority of IBM or Serenity Systems.
Neither OS/2 nor eComStation are Open Source. However, over 1500 components of them do have publically accessible source code