OGA file format
Microsoft Windows Media Player
VLC media player
OGA files are the audio-only element of the OGG multimedia file system. If you were accessing an OGA file before 2007, it would probably have appeared on your PC as a document with .ogg extension. However, since the OGG format creator, Xiph.Org Foundation, created other extensions to cater to specific data types, multimedia files are now stored with data-specific extensions like .ogv, .oga, .ogm, .ogx, all under the OGG file system. In effect, instead of having a media file containing audio, video, text, and metadata all encoded into a chunk, each data type is encoded separately with a specific extension.
Since the OGG file system is open source, the audio-only OGA files can be written or modified using Ogg FLAC, OggPCM, Ogg Vorbis, or Speex, the last two not having favorable reviews. The Xiph.Org Foundation made a similar software to the public; the software pack makes it possible for open-source or commercial Windows applications to utilize OGA files without any conflict.