Digging up old archived pages of
www.osseum.com at the "wayback machine" and other googling, this is what I found for Osseum's client list.
Auran - d20
Bad Axe Games - d20
Bard's Productions - d20
Bastion Press - d20
BlackWyrm Games - M&M
Contested Ground Studios
Dark Portal Games - d20
Eos Press
Fast Forward Entertainment - d20
Gaslight Press - d20
Ghazpork Industrial
Green Knight Publishing - Pendragon
Green Ronin Publishing - d20
Heresy Gaming
Living Imagination - d20
Louis Porter, Jr. Design - M&M
Mystic Eye Games - d20
Mythic Dreams Studios - d20
Paradigm Concepts, Inc. - d20
Perpetrated Press - d20
Red Spire Press - d20
RPG Objects - d20
Second World Simulations - d20
SkeletonKey Games - d20
Thunderhead Games - d20
Twin Rose Software - d20
Tynes Cowan Corporation
United Playtest, Inc - d20
Valar Project, Inc. - d20
Visionary Entertainment Studios, Inc.
A total of around twenty d20 3PP companies represented by Osseum Entertainment back in 2001-2004. With Osseum out of the picture by 2004 and the d20 bubble bursting, it's no big surprise many of these d20 companies dropped out of sight shortly thereafter. Osseum wasn't there anymore to convince now-skeptical distributors to carry their clients' products.