| Opalie AS | Wholesale & retail | 2026 |
| Opas Holding AS | Information & communication | 2005 |
| Opasso AS | Wholesale & retail | 2026 |
| OPBLÅ2 AS | Information & communication | 2021 |
| Opera 60 AS | Administrative & support | 2017 |
| Opheim Invest Holding AS | Real estate | 2018 |
| Opheim Malerservice AS | Construction | 2024 |
| Opisense AS | Information & communication | 2025 |
| Opor Holding AS | Real estate | 2005 |
| Opp I Ringan AS | Real estate | 2019 |
| Oppdal Alpintun Holding AS | Real estate | 2018 |
| Oppdal Eiendomsutvikling AS | Real estate | 2021 |
| Oppdal Live AS | Administrative & support | 2017 |
| Oppdalslia AS | Real estate | 2006 |
| Opplev Norge AS | Information & communication | 2018 |
| Opplæringskontoret for Bilfag AS | Education | 1987 |
| Opplæringskontoret for Frisørfag I Midt-Norge SA | Education | 1989 |
| Opportunita Holding AS | | 2012 |
| Oppoverbakke AS | Professional & scientific | 2005 |
| Opsahl Eiendom Alver AS | Real estate | 2016 |
| Opsahl Eiendom Haugesund AS | Real estate | 2020 |
| Optiflex AS | | 2024 |
| Optiker Lundemo AS | Wholesale & retail | 1986 |
| Optimal Trading AS | | 2020 |
| Optimer AS | Professional & scientific | 2021 |
| Optimil AS | Professional & scientific | 2014 |
| Optiplan AS | Information & communication | 2024 |
| Optitrails AS | Information & communication | 2023 |
| Optonor AS | Manufacturing | 2015 |
| Optonor Holding AS | Real estate | 2015 |
| Optoscale AS | Manufacturing | 2015 |
| Opus Bygg AS | Construction | 2006 |
| Ora AS | Real estate | 2023 |
| Ora Eiendom AS | Real estate | 2023 |
| Orbipitch AS | Professional & scientific | 2026 |
| Orbit AS | Professional & scientific | 2023 |
| Ord Invest AS | Financial services | 2018 |
| Orderløkken Fysioterapi AS | Health & social care | 2022 |
| Ordinary World AS | | 2023 |
| Oreid Eiendom AS | Real estate | 2016 |
| Oreid Utvikling AS | Real estate | 2006 |
| Organisk Rådgivning AS | Professional & scientific | 2016 |
| Orient Trondheim AS | Other services | 2019 |
| Origo Industrieiendom AS | | 2021 |
| Origo Legesenter AS | Health & social care | 2024 |
| Orion Bil AS | Wholesale & retail | 2025 |
| Orion Vaktmesterservice AS | Administrative & support | 2001 |
| Oris Dental AS | Health & social care | 2018 |
| Oris Dental Holding AS | Health & social care | 2016 |
| Oritro AS | Real estate | 2004 |
| Orkanger Bygg AS | Real estate | 2015 |
| Orkanger Havn AS | Real estate | 2015 |
| Orkdalsveien 30 AS | Real estate | 2018 |
| Orkdalsveien 41 Eiendom AS | Real estate | 2023 |
| Orkla Snacks Norge AS | Manufacturing | 1986 |
| Orkla Utvikling AS | Real estate | 2005 |
| Orkland Eiendom Holding AS | Real estate | 2023 |
| Orloan AS | Information & communication | 2024 |
| Ormen Langes Offisin SA | Real estate | 2017 |
| Ormen Langesv. 12 AS | Real estate | 2007 |
| Oros Invest AS | | 2026 |
| Orpahus AS | | 2023 |
| Os Naturstein AS | Mining & quarrying | 2015 |
| Oscar AS | Hospitality | 1984 |
| Oscar Tango AS | Professional & scientific | 2025 |
| Oscars Gate 56 AS | Real estate | 2023 |
| Oska Pizzeria AS | Hospitality | 2005 |
| Oskar Moe AS | Wholesale & retail | 2022 |
| Oskaroskar AS | Financial services | 2015 |
| Oslo Strategy AS | Professional & scientific | 2020 |
| Osloveien 12 A&B AS | Real estate | 2020 |
| Osloveien 15 Trondheim AS | Real estate | 1966 |
| Osloveien 16 AS | Real estate | 2017 |
| Osloveien 24 Røros AS | Real estate | 2024 |
| Osloveien 278 C AS | Real estate | 2023 |
| Osloveien 44A AS | Real estate | 2000 |
| Osloveien Utvikling AS | Real estate | 2018 |
| Osman Taxidrift AS | Transport & logistics | 2019 |
| Osn AS | Wholesale & retail | 2017 |
| Oss-To Invest AS | Health & social care | 2023 |
| Ostindiske AS | Wholesale & retail | 2022 |
| Osvald Gjetø AS | Construction | 1981 |
| Osvegen 218 AS | Real estate | 2022 |
| Otcb AS | Real estate | 2006 |
| Otei AS | Professional & scientific | 2020 |
| Otem Invest AS | | 2018 |
| Oten AS | Information & communication | 2015 |
| Other Projects AS | Professional & scientific | 2020 |
| Otium Trondheim AS | Real estate | 2005 |
| Ots AS | | 2022 |
| Ottervik Berge Holding AS | | 2019 |
| Otto Invest AS | Financial services | 2005 |
| Otto Lote AS | Information & communication | 2020 |
| Otto Nielsens Veg 12 AS | Real estate | 2006 |
| Our Invest AS | Financial services | 2005 |
| Ousland Colliard Explorers AS | Administrative & support | 2018 |
| Out There AS | Information & communication | 2026 |
| Outpost Scandinavia AS | Professional & scientific | 2014 |
| Ove Bremseth Holding AS | Real estate | 2022 |
| Over Easy Solar SPV AS | | 2025 |