osdm/OSDM = old school death metal. Used by lazy last.fm taggers in place of saying 'old school death metal'.
Serves a real-life purpose though: A way to keep scenes out of metal discussions, ie
WITHOUT ABBREVIATION:
Metalhead 1: You heard Demigod's cult debut album, Slumber Of Sullen Eyes?
Metalhead 2: Hell yeah, I have it, first press CD on drowned productions, quality 1992 old school death metal for sure! Uninformed scene kid: Death metal!? like deathcore? like Suicide Silence!? br00tal!
WITH ABBREVIATION:
Metalhead 1: You heard Demigod's cult debut album, Slumber Of Sullen Eyes?
Metalhead 2: Hell yeah, I have it, first press CD on drowned productions, quality 1992 OSDM for sure!