Oxygen

Oxygen, a colourless, inodorous gas which constitutes one-fifth in volume of the atmosphere, and which, in combination with hydrogen, forms water. It is the most widely diffused of all the elementary bodies, and an essential support to everything possessed of life.

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

Oxus * Oyer and Terminer
Owen, Sir Richard
Owen, Robert
Owens College
Oxenford, John
Oxenstiern, Axel, Count
Oxford
Oxford School
Oxford University
Oxfordshire
Oxus
Oxygen
Oyer and Terminer
Ozone
Pache, Jean
Pachomius, St.
Pachydermata
Pacific Ocean
Packhard
Pactolus
Pacuvius
Padang