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