- "Oasi Maria Santissima" Institute for Research and Prevention of Mental Retardation
- Observational and Pragmatic Research Institute, Singapore, Singapore
- Observatoire de la Côte d'Azur
- Observatoire de Lyon
- Observatoire de Paris
- Observatoire des polluants urbains [Paris]
- Observatoire des Sciences de l'Univers
- Observatoire Européen des Drogues et des Toxicomanies
- Observatoire Français de la Sclérose En Plaques [Lyon]
- Observatoire Régional de la Santé Limousin
- Observatoire régional de la santé Provence-Alpes-Côte d'Azur [Marseille]
- Observatoire Régional de Santé de Basse-Normandie
- Observatoire régional de santé Île-de-France [Paris, France]
- Obstetrics and Gynecology, Aarhus University Hospital, Aarhus, Denmark
- Ocean University of China
- Ochanomizu University
- Oc Santé [Montpellier]
- Odense University Hospital
- Odense University Hospital [Odense, Denmark]
- Oenobrands SAS
- Office du Patrimoine et de l'Archéologie Neuchatel
- Office français de la biodiversité
- Office National de la Chasse et de la Faune Sauvage
- Office National de la Chasse et de la Faune Sauvage, Direction des Etudes et de la Recherche
- Office national des forêts
- Ohio State University [Columbus]
- Okayama University
- Okinawa Institute of Science and Technology Graduate University
- Oklahoma State University [Stillwater]
- ONCFS
- Onderstepoort Veterinary Institute
- Ondokuz Mayis University
- ONERA
- ONET Technologies
- Open Cascade
- Open Society Foundations
- Open University of Catalonia [Barcelona]
- Oppelia [Paris]
- Orange (France)
- Orange Labs
- Orange Labs Networks
- Orano Canada Inc
- Orano Projets
- Orano Recyclage
- Örebro University
- Örebro University Hospital [Örebro, Sweden]
- Oregon Health and Science University [Portland]
- Oregon State University
- Organisation Bretonne de Sélection
- Organisation de Coopération et de Développement Economiques = Organisation for Economic Co-operation and Development
- Organisation Mondiale de la Santé | World Health Organization Office
- ORL, CHRU Brest, Brest, France
- OROVEL
- ORS PACA
- Osaka City University
- Osaka Prefecture University
- Osaka Research Institute of Industrial Science and Technology
- Osaka University [Osaka]
- Oslo Metropolitan University
- Oslo University Hospital [Oslo]
- Ospedale Policlinico San Martino [Genoa]
- Ospedale San Camillo-Forlanini
- OST
- Otto Bock Healthcare GmbH
- Otto-von-Guericke-Universität Magdeburg = Otto-von-Guericke University [Magdeburg]
- Oulu University Hospital [Oulu]
- Oviatis SA
- Ovidius University of Constanta
- Ovoteam
- Owkin France
- Oxford Brookes University
- Oxford University Clinical Research Unit [Ho Chi Minh City]