| Old Money Ventures AS | Professional & scientific | 2022 |
| Oldeidegården AS | Real estate | 2005 |
| Ole J Todnems Invest AS | Real estate | 1977 |
| Ole Klev Consulting AS | Professional & scientific | 2012 |
| Ole S Simonsen AS | Real estate | 2021 |
| Ole Simon AS | Wholesale & retail | 1992 |
| Olive Tree AS | Hospitality | 2016 |
| Olo Capital AS | | 2026 |
| Olsen & Anfindsen Invest AS | | 1996 |
| Olsens Service AS | Administrative & support | 2020 |
| Ombo Eiendom Da | Real estate | 2005 |
| Omega Trygg CFD KS | | 2012 |
| Ommang Invest AS | Information & communication | 2006 |
| Ommundsen Holding AS | Health & social care | 2019 |
| Ommundsen Øre-Nese-Hals AS | Health & social care | 2018 |
| Omniflow AS | Professional & scientific | 2026 |
| Omnio AS | Professional & scientific | 2018 |
| Omoo AS | Information & communication | 2025 |
| Onesmart AS | Information & communication | 2019 |
| Onesource AS | Manufacturing | 2022 |
| Onkel Folke Da | Arts & recreation | 2005 |
| Ono Consulting AS | Professional & scientific | 2016 |
| Ono Investment AS | Real estate | 2023 |
| Ont Holding AS | | 2026 |
| Ontoson AS | Information & communication | 2016 |
| Ooa Invest AS | | 2023 |
| Ooy Thaimassasje AS | Other services | 2019 |
| Opal AS | Wholesale & retail | 2002 |
| Opamp Engineering AS | Professional & scientific | 2012 |
| OPB1 AS | Real estate | 2013 |
| Opdam Invest AS | | 2025 |
| Openhub AS | Information & communication | 2026 |
| Opes Capital AS | | 2026 |
| Opor Consulting AS | Professional & scientific | 2026 |
| Oppbakken AS | Professional & scientific | 2021 |
| Opr Invest AS | | 2024 |
| Opteo AS | Construction | 2018 |
| Opteo Eiendom AS | Real estate | 2024 |
| Optikk Holding AS | Real estate | 2005 |
| Optima Eiendom AS | Administrative & support | 2024 |
| Optimal Trade AS | | 2024 |
| Optimi Consulting AS | Professional & scientific | 2014 |
| Option Holding AS | Financial services | 2016 |
| Opus Eiendom AS | Real estate | 1993 |
| Opus Legesenter AS | Health & social care | 2014 |
| Orakel Oddsconsult AS | Financial services | 2005 |
| Oranje Betongvarer Norge Ans | Wholesale & retail | 1988 |
| Orcana AS | | 2022 |
| Oregano Da | Information & communication | 2008 |
| Orenda Holding AS | | 2025 |
| Orh Investering AS | Professional & scientific | 2020 |
| Orient Hsse AS | Professional & scientific | 2017 |
| Oriental Union AS | Hospitality | 2007 |
| Orientstil AS | Wholesale & retail | 2021 |
| Origin AS | Information & communication | 2017 |
| ORIGIN3 AS | Information & communication | 2021 |
| Origo Consulting AS | Professional & scientific | 2008 |
| Ormaskogen Barnehage SA | Education | 2002 |
| Oro AS | Other services | 2019 |
| Ors Systemer AS | Professional & scientific | 2020 |
| Orstad Utviklingsselskap AS | Real estate | 2014 |
| Orstadveien 108 Klepp AS | Real estate | 2010 |
| Orstadveien 110 AS | Real estate | 2022 |
| Os Transport AS | Transport & logistics | 2023 |
| Osim AS | | 2021 |
| Oskar Consult AS | Wholesale & retail | 1988 |
| Osli Eiendom AS | Real estate | 2013 |
| Osli Opeldeler Ans | Wholesale & retail | 2007 |
| Osliveien AS | Real estate | 2026 |
| Oslofjordklinikken Vest AS | Health & social care | 2014 |
| Osmundsen AS | Financial services | 2023 |
| Osse AS | | 1982 |
| Otech Consult AS | Information & communication | 2023 |
| Otg AS | | 2001 |
| Otica AS | Real estate | 1979 |
| Otteren Gullegget AS | Wholesale & retail | 1987 |
| Otteren Invest AS | Real estate | 2016 |
| Otteren Jewellery AS | Manufacturing | 2023 |
| Ottes Holding AS | | 2023 |
| Otto-Holding AS | | 2026 |
| Ov Holding AS | Financial services | 2005 |
| Ove Lunde Holding AS | Wholesale & retail | 2005 |
| Overflow Energies AS | Energy | 2026 |
| Ovstro Invest AS | | 2025 |
| OY E-Qual Partners Ltd | Health & social care | 2015 |
| P & P Facility Services AS | Administrative & support | 2013 |
| P G Andersen & Bruhn Orgelbyggeri Aps | Other services | 1995 |
| Pagi Invest AS | | 2019 |
| Palle Miljø AS | Water & waste | 2024 |
| Palmar Andre Generasjon AS | Information & communication | 2020 |
| Paltek AS | Other services | 2006 |
| Panasonic Marketing Europe GmbH Nuf | Wholesale & retail | 2011 |
| Panczel Medical AS | Health & social care | 2024 |
| Panorama Energy AS | Information & communication | 2026 |
| Papilotten Hår Ans | Other services | 1993 |
| Pario Invest AS | Real estate | 2015 |
| Parkering Og Hotell AS | Transport & logistics | 2005 |
| Parm AS | Administrative & support | 2022 |
| Pars Invest AS | Other services | 2010 |
| Part Owners Holding AS | Professional & scientific | 2020 |