olefins
oligo
oligonucleotides
oligopeptides
oligosaccharides
onium compounds
organo-
organoheteryl groups
organometallic compounds
organyl groups
ortho acids
ortho amides
ortho esters
osazones
osides
osones
osotriazoles
oxenium ions
oxime O-ethers
oximes
oxo acids
-oxo carbenes
oxo carboxylic acids
oxo compounds
oxoacids
oxocarbons
oxonium ions
oxonium ylides
oxyacid/oxy-acid
oxylium ions
ozonides