An OGM file is a Ogg Media File. Unlike typical avi's, OGM's have the capability to support multiple audio and video tracks. So you can switch video and audio content both, example: switching between English and Japanese Audio. You may also embed extra features like subtitles and extra information into OGM files without the need of a seperate subtitle script file.
There are many players which suppport the playback of OGM's, such as Media Player Classic, BSPlayer, etc. Google these terms to look for where to download them. I believe ordinary WMP and Winamp should also be able to play these formats if you have the necessary codecs installed which will allow these players to decode all tracks available inside that particular .ogm file which you are trying to watch.
Universally, a standard recommendation would be to install the following:
- XviD Codec
- Subtitle Filter (you don't need this if you have Media Player Classic)
- DirectVobSub
- Directshow Filter (OGG)