Old School Death Metal

Try Morbid Angel - Altars Of Madness if you haven't heard it.

If that's the only one you've heard. Go with Covenant.

Obituary - Cause Of Death
At The Gates - The Red In The Sky Is Ours
Brutality - Screams Of Anguish
Cryptosy - None so Vile (probably not considered OSDM, more on the tech side)
Grave - Into The Grave
 
it took me months to get into morbid angel but now they're one of my favorites.

a great old school death metal CD is Bloodbath's first EP Breeing Death. it's not actually old school, but it imitates it
 
Crown of Thorns- The Burning
Crown of Thorns- Eternal Death
Desultory- Into Eternity
Desultory- Bitterness
Unanimated- Ancient God of Evil
Atrocity- Longing for Death
 
Personal favorites (in no particular order):

Unleashed - Shadows in the Deep
Therion - Beyond Sanctorum
At The Gates - The Red in the Sky is Ours
Amorphis - The Karelian Isthmus
Carnage - Dark Recollections
Darkthrone - Soulside Journey
God Macabre - The Winterlong
Immolation - Dawn of Possession
Sinister - Cross the Styx
Utumno - Across the Horizon
Demigod - Slumber of Sullen Eyes
Demilich - Nespithe
Adramelech - Psychostasia
Crematory - Denial
Sorcery - Bloodchilling Tales
Asphyx - The Rack
Pestilence - Consuming Impulse
Bolt Thrower - War Master
Carbonized - For the Security
Cemetary - An Evil Shade of Grey
Convulse - World Without God
Divine Eve - Upon These Ashes Scorn the World
Epitaph - Seeming Salvation
Deicide - Deicide
Purtenance - Member of Immortal Damnation