Organic molecules, b. [460].
Organization, b. [435].
Oscillation, Centre of, a. [356].
Outcrop (of strata), b. [512].
Oxide, b. [282].
Oxyd, b. [282].
Oxygen, b. [276].
Palæontology, b. [519].
Palætiological sciences, b. [499].
Parallactic instrument, a. [165].