AAC is a sound encoded file format developed by Fraunhofer IIS, AT&T, Dolby and Sony. Its name stands for Advanced Audio Coding. The AAC format is an industry standard and was created as an improved alternative to the most common MP3 audio file format. AAC files do have slightly improved features compared to MP3s. Let's say this format provides the opportunity for improved processing of frequencies above 16 kHz. At the same time, the sound quality of AAC files is closer to the original sound source.
OGA is a format of free container audio files, a kind of audio container. The developer is Xiph.Org. The full original name is Ogg Vorbis Audio File. This audio container runs on Ogg Vorbis, a widely used audio encoding and streaming technology. The container stores audio data with compression using Vorbis compression. OGA audio files can also use the .OGG extension because they are a standard Ogg container. Any Ogg-enabled media player must play OGA audio files.