| Onecar AS | Wholesale & retail | 2025 |
| Onsøyen Invest AS | Financial services | 2007 |
| Oord Trefelling AS | Agriculture & fishing | 2026 |
| Opal Ans | Hospitality | 1993 |
| Openstudio AS | Professional & scientific | 2025 |
| Opn Holding AS | Real estate | 2015 |
| Opn Multiservice AS | Professional & scientific | 2024 |
| Oppsann AS | Financial services | 2011 |
| Optiker L Eriksen AS | Wholesale & retail | 1928 |
| Optiker Wøllo AS | Wholesale & retail | 1988 |
| Optimisten Rengjøringsbyrå Da | Administrative & support | 1996 |
| Optimus-Klinikken AS | Other services | 2014 |
| Optiwin Software AS | Information & communication | 2000 |
| Opturo AS | Professional & scientific | 2026 |
| Orakel Kapital AS | Hospitality | 2018 |
| Oranus AS | | 2024 |
| Ord & Bild Da | Arts & recreation | 1995 |
| Order Control AS | Information & communication | 2019 |
| Org Holding AS | | 2025 |
| Organic Baby AS | | 2023 |
| Orhun Holding AS | Financial services | 2014 |
| Orhusveien 9 AS | Real estate | 2018 |
| Origo Eiendom AS | Real estate | 1996 |
| Origo Leilighetshotell AS | Real estate | 1974 |
| Origo One Step AS | Real estate | 2018 |
| Origo ST. Olavsgate 2 AS | Real estate | 2020 |
| Origo ST. Olavsgate AS | Real estate | 2020 |
| Origo Utleie AS | Real estate | 2004 |
| Origo Øvre Storgate 3 AS | Real estate | 2020 |
| Origo Øvre Torggate AS | Real estate | 2007 |
| Ortopedia AS | Health & social care | 2007 |
| Orya Holding AS | | 2025 |
| Osaka Sushi Drammen AS | Hospitality | 2014 |
| Oseberg Invest AS | Real estate | 1998 |
| Oslo Fraktservice AS | Transport & logistics | 2024 |
| Oslo Medical AS | Wholesale & retail | 2006 |
| Oslo Og Drammen VVS AS | Construction | 2026 |
| Oslo Sveiseservice AS | Manufacturing | 2023 |
| Oslo Tak & Bygg AS | Construction | 2024 |
| Oslo Vask AS | Administrative & support | 2014 |
| Oslofjord Bedriftsrådgivning AS | Professional & scientific | 2014 |
| Oslofjord Invest AS | | 2005 |
| Oss Norge AS | Information & communication | 2020 |
| Ossikel AS | Real estate | 2003 |
| Ostmont AS | Administrative & support | 2014 |
| Osw AS | | 2016 |
| Otea AS | | 2018 |
| Othesa AS | Health & social care | 2019 |
| Otorhino AS | Real estate | 1986 |
| Otry AS | | 2023 |
| Ottesen Invest AS | Financial services | 2005 |
| Otto Christensen & Kaj Sørensen AS | | 1995 |
| Oui Clothing AS | Wholesale & retail | 2020 |
| Outline Arkitektur AS | Professional & scientific | 2023 |
| Outline Holding AS | Real estate | 2025 |
| Outline-Ark Eiendom AS | Real estate | 2025 |
| Ove Byrmo Holding AS | Real estate | 1992 |
| Ovenstadveien AS | Real estate | 2020 |
| Ozi Frisør AS | Other services | 2022 |
| P&A Eiendom AS | Construction | 2026 |
| P.A.Eiendom AS | Real estate | 2005 |
| P.I.P - Bygg AS | Construction | 2017 |
| Packamist AS | | 2024 |
| Packline Eiendom AS | Real estate | 2010 |
| Pacla AS | Financial services | 2017 |
| Padel AS | Real estate | 2020 |
| Padel for Alle AS | | 2021 |
| Padel Import AS | Wholesale & retail | 2023 |
| Paf Software AS | Information & communication | 2020 |
| Pajomi AS | | 2016 |
| Palito Holding AS | Health & social care | 2017 |
| Palmberg Bilverksted AS | Other services | 1995 |
| Palmberg Takst AS | Real estate | 2010 |
| Palmereiser AS | Administrative & support | 2026 |
| Palmette AS | Professional & scientific | 2025 |
| Panelbygg AS | Construction | 1997 |
| Panelbygg Eiendom AS | Real estate | 2008 |
| Panmi AS | | 2021 |
| Panorama Musikk Drammen AS | Wholesale & retail | 1988 |
| Panorama Utvikling Telemark AS | Real estate | 2018 |
| Panoramagutta AS | Education | 2024 |
| Papertek Forlag AS | Information & communication | 2025 |
| Papp AS | Wholesale & retail | 2005 |
| Paradise International Service AS | Administrative & support | 2012 |
| Paragon Arrays AS | Wholesale & retail | 2005 |
| Paratus Media AS | Arts & recreation | 2001 |
| Parc Trading AS | Wholesale & retail | 2025 |
| Park & Kombimaskiner AS | Wholesale & retail | 2023 |
| Park Utvikling AS | Real estate | 2013 |
| Park Utvikling II AS | Real estate | 2013 |
| Parkettproffen AS | Construction | 2022 |
| Parking Guru AS | Information & communication | 2012 |
| Partner Drift AS | Construction | 2021 |
| Pasdar Holding AS | | 2026 |
| Pasio AS | Real estate | 2005 |
| Paski Invest AS | | 2022 |
| Paul AS | Wholesale & retail | 1989 |
| Pavarotti Eiendom AS | Real estate | 2001 |
| Pavarotti Restaurant AS | Hospitality | 2025 |
| Pax Invest AS | Financial services | 2012 |