| Oslo Internasjonale Teaterfestival | Oslo Internasjonale Teaterfestival | 1989 | 3 | |
| Orchestra del Teatro Lirico di Cagliari | Orchestra del Teatro Lirico di Cagliari | 2024 | 1 | |
| Oda og Martinius Nielsen-turnéen | Oda og Martinius Nielsen-turnéen | 1895 | 15 | |
| Omada Theatrou X-ACT ART | Omada Theatrou X-ACT ART (X-ACT ART Theater Group) | 2024 | 1 | |
| Oxford Players, The | The Oxford Players | 1924 | 3 | |
| Os Comediantes de Lisboa | Os Comediantes de Lisboa | 1947 | 1 | |
| Ole Morten Øwre Storhaug | Ole Morten Øwre Storhaug | 2012 | 1 | |
| Onafhankelijk Toneel | Onafhankelijk Toneel | 1998 – 2009 | 27 | |
| Oslo Katedralskoles festsal | Oslo Katedralskoles festsal | 2003 | 1 | |
| Orion Pictures | Orion Pictures | 2025 | 1 | |
| Orchestra dell'Accademia Nazionale di Santa Cecilia | Orchestra dell'Accademia Nazionale di Santa Cecilia | 1979 | 1 | |
| Ottilia Littmarcks sällskap | Ottilia Littmarcks sällskap | 1880 – 1899 | 10 | |
| Orchestra del Maggio Musicale Fiorentino | Orchestra del Maggio Musicale Fiorentino (Maggio Orchestra) | 2024 | 1 | |
| Over the Moon Productions | Over the Moon Productions | 2010 | 1 | |
| Other Eye of The Tiger | The Other Eye of The Tiger | 2023 | 1 | |
| Otterbein University Theatre | Otterbein University Theatre | 2011 | 1 | |
| Otto Jacobsen Tourneen | Otto Jacobsen Tourneen | 1917 | 1 | |
| Oregon State University Theatre | Oregon State University Theatre | 2005 | 1 | |
| Oslo Barne & Ungdomsteater | Oslo Barne & Ungdomsteater | 2006 | 1 | |
| Opera Telemark | Opera Telemark | 1997 | 1 | |
| Orchestra del Teatro Carlo Felice | Orchestra del Teatro Carlo Felice | 2006 | 1 | |
| Orleneff’s Russian Dramatic Company | Orleneff’s Russian Dramatic Company | 1906 | 3 | |
| Orchestra del Teatro Massimo Bellini | Orchestra del Teatro Massimo Bellini | 2024 | 2 | |
| Olaus Olsens Theaterselskab | Olaus Olsens Theaterselskab | 1883 – 1904 | 77 | |
| Oslo Fringe festival | Oslo Fringe festival | 2023 – 2024 | 2 | |
| Old Tote Theatre Company, The | The Old Tote Theatre Company | 1967 – 1976 | 4 | |
| Oxford Union Dramatic Society | Oxford Union Dramatic Society | 1922 | 1 | |
| Oasis Theatre Company | Oasis Theatre Company | 1991 – 1994 | 3 | |
| OBAMA-festivalen | OBAMA-festivalen | 2009 | 1 | |
| Oslo Moderne Teater | Oslo Moderne Teater | 2002 | 2 | |
| Orchestra Sinfonica di Torino della RAI | Orchestra Sinfonica di Torino della RAI | 1971 | 1 | |
| Oslo Nye Teater (Centralteatret) | Oslo Nye Teater (Centralteatret) | 1966 – 2009 | 20 | 2 |
| OFF Escuela de Teatro y Cine | OFF Escuela de Teatro y Cine | 2008 | 4 | 4 |
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