| Oar Eiendom AS | Real estate | 2024 |
| Obilink AS | | 2025 |
| Obsido AS | Financial services | 2006 |
| Obsify AS | Health & social care | 2026 |
| Oc Consulting AS | Professional & scientific | 2020 |
| Occ Finance & Accounting AS | Professional & scientific | 2018 |
| Ocean Aquafarms AS | Manufacturing | 2015 |
| Ocean Energies AS | Professional & scientific | 2020 |
| Ocean Installer AS | Mining & quarrying | 2011 |
| Ocean Installer Equipment AS | Administrative & support | 2012 |
| Ocean Installer Holding AS | Mining & quarrying | 2015 |
| Ocean Installer II AS | Mining & quarrying | 2010 |
| Ocean Services AS | Professional & scientific | 2007 |
| OCEAN24 AS | Information & communication | 2022 |
| Oceanway Invest AS | Professional & scientific | 2021 |
| Octocode | Education | 2025 |
| Octopus Analytica AS | Information & communication | 2026 |
| Odd Berge Førland Holding AS | Real estate | 2019 |
| Odd Fellow-Gården AS | Real estate | 1983 |
| Odd Group AS | Hospitality | 2023 |
| Odd Standard AS | Manufacturing | 2014 |
| Odden 1 AS | Real estate | 2023 |
| Oddvar Holding AS | | 2023 |
| Odel Og Eie AS | Real estate | 2020 |
| Odel-Ai AS | Information & communication | 2025 |
| Odeon Kino Stavanger/Sandnes AS | Information & communication | 2000 |
| Odin Eiendom AS | Real estate | 1969 |
| Odw Invest AS | | 2022 |
| Oere Invest & Consulting AS | Professional & scientific | 2015 |
| Office Stavanger AS | Real estate | 2017 |
| Offshore & Marine AS | Real estate | 2005 |
| Offshore Energy Construction AS | Transport & logistics | 2022 |
| Offshore Equipment Hub AS | Wholesale & retail | 2022 |
| Offshore Flotation AS | Wholesale & retail | 1997 |
| Offshore Link AS | Professional & scientific | 2019 |
| Offshore Marine Technology AS | Construction | 2023 |
| Offshore Marine Technology Engineering AS | Administrative & support | 2024 |
| Offshore Marine Technology Holding AS | Construction | 2024 |
| Offshore Qualific AS | Professional & scientific | 2022 |
| Offshore-Energy AS | Professional & scientific | 2026 |
| Offside Investering AS | Real estate | 2017 |
| Ofl Holding AS | Financial services | 2018 |
| Oftedahls Auto AS | Real estate | 1970 |
| Oftedahls Auto Holding AS | Real estate | 2005 |
| Oftedal AS | | 2025 |
| Oftedal Revisjon & Rådgivning AS | Professional & scientific | 2023 |
| Oge AS | | 2024 |
| Ogj Holding AS | | 2026 |
| Ognedal Holding AS | | 2024 |
| Ogoori AS | Administrative & support | 2019 |
| Oh My Garn AS | Wholesale & retail | 2023 |
| Ohana AS | Financial services | 2017 |
| Ohana Invest AS | Real estate | 2008 |
| Ohf Trading AS | Wholesale & retail | 2021 |
| Ohm Invest AS | Real estate | 2006 |
| Ohm Investment AS | | 2021 |
| Ohmega Eiendom AS | Real estate | 2015 |
| Ohmega Holding AS | Professional & scientific | 2011 |
| Ohmtech Invest AS | Real estate | 2005 |
| Oholding AS | Financial services | 2011 |
| Ohop Holding AS | Construction | 2020 |
| Ohoy Systems AS | Professional & scientific | 2018 |
| Ohu Invest AS | Real estate | 2016 |
| Oi Kapital AS | | 2019 |
| Oik Completion AS | Professional & scientific | 2022 |
| Oil Consultants Norway AS | Professional & scientific | 2020 |
| Oilers Esport AS | Arts & recreation | 2020 |
| Oilers Event AS | Professional & scientific | 2017 |
| Oilers Holding AS | Arts & recreation | 2010 |
| Oilers Invest AS | Real estate | 2010 |
| Oilfield Production Consultants (Opc) Norge AS | Professional & scientific | 2013 |
| Oim Invest AS | | 2017 |
| Ois PLC | Mining & quarrying | 1995 |
| Ojn Invest AS | Professional & scientific | 1998 |
| Ojs Investments AS | | 2023 |
| Okio AS | Real estate | 2007 |
| Okr Finans AS | Financial services | 2005 |
| Oks Consulting AS | Professional & scientific | 2019 |
| Okse AS | Information & communication | 2013 |
| Okutan Group AS | Real estate | 2025 |
| Okvas Eiendom AS | Real estate | 2021 |
| Ol Offshore AS | Professional & scientific | 2020 |
| Ola SR Holding AS | Real estate | 2024 |
| Olav Beisland AS | Information & communication | 2005 |
| Olav Stangeland AS | | 2013 |
| Olav V'S Gate 11 Holding AS | Real estate | 2019 |
| Olav Wigestrand AS | Construction | 2024 |
| Olavskleivå AS | Real estate | 2026 |
| Olavstoppen AS | Information & communication | 2010 |
| Oldeide Invest AS | Financial services | 2005 |
| Olden Holding AS | Financial services | 2015 |
| Ole Andre Torjussen Holding AS | Information & communication | 2012 |
| Ole Dolven Consulting AS | Professional & scientific | 2025 |
| Ole Marcus Rasmussen AS | Financial services | 2017 |
| Ole Melberg Management AS | Financial services | 1998 |
| Ole Oppistov AS | Construction | 2016 |
| Olehus AS | Construction | 2016 |
| Olehus Montasje AS | Construction | 2024 |
| Olesen Consult Hvac AS | | 1997 |
| Oleum Group AS | Information & communication | 2026 |