(historical) a halfpenny

(archaic) an objection / (genetics) the obese gene

Abbreviation of obstetric. / (golf) Initialism of out of bounds. / Abbreviation of obligatory. (as in "ob. link" on Usenet) (see ObLink at c2.com; see also ob-) / Abbreviation of Old Babylonian.

(sports) Initialism of Olympic best. / (television, radio) Initialism of outside broadcast. / Initialism of old boy. (male graduate; also a CB radio operator's term of address for a male)

(music) Abbreviation of oboe.

A river in Russia; the westernmost of the three great Siberian rivers.

(category theory) The set of objects of a category.

(non-productive) Against; facing; a combining prefix found in verbs of Latin origin. / (botany) Of a reversed shape.

(Internet, informal, humorous) Obligatory; prepended to the name of a topic being mentioned to avoid accusations of being off-topic.

(in listing dates of death) Died.