- O.T.H. Batiments
- Oak Structural Design Office
- OAKh
- Claude Henry Oakland
- Oakland and Imada
- Oakland, Claude, and Associates, Architects
- Charles Warren Oakley
- ÖBA S35, Projektgemeinschaft Geoconsult / Spirk und Partner
- Gyo Obata
- Gyo Obata
- Obayashi
- Obayashi Corporation
- Obayashi Corporation
- ÖBB
- Friedrich Oberauer
- Fritz Oberauer
- Artur Oberbichler
- Oberdorfer
- Oberhauser & Schedler Bau GesmbH & Co
- Alfons Oberhofer
- Günther Oberhofer
- Florian Oberhuber
- Oswald Oberhuber
- H. Peter Oberlander
- Wolfgang Oberlik
- Obermayr Holzkonstruktionen GmbH
- Obermeyer Planen + Beraten
- Ober-Mörlen
- Johann Obermoser
- Obermoser arch-omo ZT Gmbh
- Jakob Oberpriller
- Oberressl & Kantz
- Gualtiero Oberti
- Andreas Oberwalder
- Jakob Oberwalder
- Veronika Oberwalder
- Zita Oberwalder
- Agusti Obiols
- Rafael Obregón Gonzalez
- José María Obregón Rocha
- Carlos Obregón Santacilia
- Obregón y Valenzuela
- O'Brien
- Arthur L. O'Brien
- Matthew O'Brien
- Smith O'Brien
- O'Brien and O'Brien, Architects
- O'Brien and Werner, Architects
- O'Brien, Smith Architect
- Clayton O'Brien-Smith
- Hans-Ulrich Obrist
- Observatorium
- Robert Obsieger
