OGA files are essentially just the audio only element of an OGG file. It's a container format that contains the audio transport stream encoded in the Vorbis format. This is the preferred extension for OGG audio only files.
The Ogg format was created as a framework which are available free of charge and re-implementable in software. The format consists of chunks of data referred to as Ogg Pages. A BSD licensed library called LibVorbis is used to encode and decode the data from the Vorbis streams. Currently there is no official standard for including metadata in Ogg containers, but must be included in the Codec.
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