Science in the 19th Century Periodical

Observatoire de Paris

Observatorio Astronómico de Bogotá

Observatorio Real de Cadiz

Observatorio Real del Madrid

Œcumenical Council, Rome

Old Bailey Sessions Court

Ophthalmic Institution, Regent's Park

Ophthalmological Society of the United Kingdom

Optische Fabrik M. Baader

Ordnance Survey of Ireland

Ordnance Survey of the United Kingdom

Orthopaedic Institution

Ospedale Gli Incurabili (Saint Maria del Popolo), Naples

Osservatorio ed Archivio Centrale Geodinamico, Rome

Overend, Gurney, and Co, firm