An OST (abbreviation of "Original Soundtrack") is a commercially released soundtrack album, featuring the music contained within a movie, TV show, videogame, etc. Only the main titles of the Metal Slug series have received official OST releases so far.
Official Metal Slug OSTs
Metal Slug 2 Original Sound Track
Main article: Metal Slug 2 Original Sound Track
Metal Slug 3 Original Sound Track
Main article: Metal Slug 3 Original Sound Track
Metal Slug 5 Original Sound Track
Main article: Metal Slug 5 Original Sound Track
Metal Slug 7 ~iTunes Limited Edition~
Main article: Metal Slug 7 ~iTunes Limited Edition~
Metal Slug Complete Sound Box
Main article: Metal Slug Complete Sound Box
This is a compilation album, featuring music from all arcade releases plus Metal Slug 3D.