Obsidian

Obsidian, a hard, dark-coloured rock of a glassy structure found in lava, which breaks with conchoidal fracture.

Definition taken from The Nuttall Encyclopædia, edited by the Reverend James Wood (1907)

Obscurantist * Occam
[wait for the fun]
Oberlin, Jean Friedrich
Oberon
Obi
Objective
Oblates
Oboe
Obelus
O'Brien, William
O'Brien, William Smith
Obscurantist
Obsidian
Occam
Occasionalism
Oceania
Oceania
Oceanides
Oceanus
Ochils
Ochiltree, Edie
Ochino, Bernardino
Ochterlony, Sir David