O

Object colour

Object colour space

Off shade

Opacity

Ophthalmoscope

Opponent colour theory
Ewald Hering (1834-1918) devised the Opponent Colour Theory based on the fact that four unique hue sensations - redness, yellowness, green-ness and blueness are required to define hue differences, and no pairs of these can be experienced simultaneously - redness+green-ness, yellowness+blueness.

OSA
The Optical Society of America.

Ostwald colour system