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!

That first part was written in 2014, and it's obviously a product of its time. However, it's still useful after a full decade here in 2024, a time in which every tag is being clogged with pop artists like Taylor Swift and cookie-cutter K-Pop boy bands. This page shows actual old school death metal artists, whereas the page for "old school death metal" is just full of stuff that got tagged as such because people think it's funny to make it harder to find music.