Observable: Some physical property of a system that can be observed.

Octet Rule: This states that "each atom in a molecule acquires shares in electrons until its valence shell achieves eight electrons".

Olefin: Old word for alkene.

Open System: A system that can exchange both matter and energy with the surroundings.

Operator: An instruction to carry out a mathematical operation upon a function.

Optically Active: Optically active molecules will rotate plane-polarised light. This is associated with asymmetry in the molecule. It is very often the result of a chiral centre.

Orbital Approximation: This states that the wavefunction for a species, atom or molecule, as a whole may be described as the product of the individual one-electron wavefunctions for the electrons contained in that species.

Orbital: An orbital is a mathematical expression of a 3D geometry where the electron is most likely to be found.

Organometallic: A compound in which metal ions are directly bonded to organic, carbon containing, species.

Orthogonal: Two planes that are at right angles to each other are said to be orthogonal.

Ortho: Substituents which are ortho to each other are next to each other. Commonly used to assign a position on an phenyl ring, relative to a substituent (X) (bear in mind this 'substituent' could actually be the rest of the molecule!);

Outer-Sphere Mechanism: An electron transfer reaction which proceeds without the two ions forming bonds to a common shared ligand.

Overlap density: The electron density where two atomic orbitals overlap. Electron density in such a region may be enhanced if the two orbitals interfere constructively, or reduced if they interfere destructively.

Overpotential: This is the extra energy required in addition to the reduction potential in order for a reaction to proceed. It reflects a kinetic barrier to the reaction.

Overtone: A line in a vibrational spectrum arising from a formally forbidden transition. They arise because anharmonicity in the potential curve of a vibrating bond can relax the selection rules for vibrational transitions.

Oxidation: In general terms, the loss of electrons. Also commonly defined as the gain of oxygen or the loss of hydrogen.