Q:What is an OGM file ?
A:Ogg Media File (OGM)

Video
The Video part of the OGM file has been encoded with the Xvid codec.
(Xvid = DivX backwards) and is an open source video codec. DivX 4 was free until it was finally released as DivX 5. Now there is further development of the Xvid codec.

Since DivX owns the trademark it could not be used on this open source codec.
DivX 3.11 alpha is a different codec, and not to be confused with DivX (Note the smiley)

Audio
The audio part of the OGM files has been encoded with Ogg Vorbis. The main reason for its development is to provide a free codec to encoded audio, as MP3 is owned by Thomson and Fraunhofer who want royalties for any MP3 file made.

Playback
Most people just rename the file from .ogm to .avi and play as normal, so long as you have both Xvid and Vorbis installed you should not have any problems with Window Media Player.

Other Players
BS Player
Zoom Player

Download
Ogg Package (Ogg Splitter, Ogg Multiplexer, Vorbis Encoder and Vorbis Decoder)
Ogg Package
Xvid (XviD-04102002-1.exe (411kb)

Problems
Xvid, as you know can be a problem, especially when used to share files. Many systems cannot use this codec at all, Xvid is still under development, and is not yet fully stable.

OGG Vorbis, not yet popular as MP3, when sound is made with this codec using a variable bit rate (VBR) and the downloaded file has corrupt frames, it becomes more difficult to fix problems. (See Vdub VBR warning)

Xvid and Vorbis are free to use.