| Oht Verksted Teknikk AS | Other services | 2026 |
| Oi! Trøndersk Mat Og Drikke AS | Arts & recreation | 2005 |
| Oil Consulting AS | Professional & scientific | 2013 |
| Oir Holding AS | Real estate | 2011 |
| Oj Aske Holding AS | | 2019 |
| Oj Lenes & Co AS | Agriculture & fishing | 2015 |
| Ojrd AS | Professional & scientific | 2013 |
| Ok Partners AS | Professional & scientific | 2026 |
| Ok-Holding AS | Construction | 2005 |
| OK26 Holding AS | | 2022 |
| Okala Holding AS | Professional & scientific | 2018 |
| Okbas Transport AS | Transport & logistics | 2014 |
| Okea ASA | Mining & quarrying | 2015 |
| Okeanos.Net AS | Information & communication | 2009 |
| Okel AS | Real estate | 2012 |
| Okel Bolig AS | Real estate | 2020 |
| Okeli AS | Real estate | 2005 |
| Okey Music AS | Arts & recreation | 2021 |
| Okkenhaug Bil AS | Wholesale & retail | 1986 |
| Okkenhaug Eiendom AS | Real estate | 2008 |
| Okkenhaug Holding AS | Real estate | 2023 |
| Okkenhaug Krogstad AS | Wholesale & retail | 2024 |
| Oklia AS | Financial services | 2009 |
| Okstad Felt C AS | Real estate | 2011 |
| Okstad Park AS | Real estate | 2011 |
| Oktagon Eiendom AS | Real estate | 2020 |
| Oktagon Holding AS | Real estate | 2020 |
| Oktagonklinikken AS | Other services | 2023 |
| Ola Auran Invest AS | Health & social care | 2023 |
| Ola Dahlsveen AS | Real estate | 1968 |
| Ola M Raavand AS | Professional & scientific | 2012 |
| Ola Setroms Veg Utvikling AS | Real estate | 2019 |
| Olabnor AS | Real estate | 2012 |
| Olaf Hansen Eiendom AS | Real estate | 1949 |
| Olaussen Holding AS | | 2019 |
| Olaussen Invest & Consulting AS | Professional & scientific | 2020 |
| Olav Egeland Invest AS | Financial services | 2005 |
| Olav Engelbrektsons Alle 51-57 AS | Households | 1956 |
| Olav Magne Gorseth AS | Construction | 2017 |
| Olav Tryggvason Holding AS | Wholesale & retail | 2002 |
| Olav Tryggvasons Gate 30 AS | Real estate | 2017 |
| Olav Tryggvasons GT 1 AS | Real estate | 2006 |
| Olav Tryggvasonsg 24 AS | Real estate | 1995 |
| Olav Tryggvasonsg 35 AS | Real estate | 1982 |
| Olav Tryggvasonsgt 29/31 AS | Real estate | 2017 |
| Olav Tryggvasonsgt 33 AS | Real estate | 1995 |
| Olavs Pub Og Kjøkken AS | Hospitality | 2026 |
| Olavsbuen AS | Real estate | 2007 |
| Olavshallen AS | Real estate | 1988 |
| Olavskvartalet AS | Real estate | 2020 |
| Old Mill AS | Manufacturing | 2025 |
| Olden AS | Professional & scientific | 2016 |
| Oldenborg Consulting AS | Real estate | 2005 |
| Oldenborg Eiendom AS | Real estate | 2006 |
| Ole Aas AS | Wholesale & retail | 1874 |
| Ole Brumms Hage AS | Real estate | 2023 |
| Ole Pettersen Eiendomsselskap AS | Real estate | 1993 |
| Ole Sivertsen AS | Construction | 1976 |
| Ole-Andreas AS | | 2022 |
| Oleg Eiendom AS | Real estate | 1958 |
| Olem Holding AS | | 2025 |
| Oleum AS | Financial services | 2005 |
| Olevi Holding AS | Professional & scientific | 2016 |
| Olex AS | Information & communication | 1997 |
| Olfa Holding AS | | 2005 |
| Olien Holding AS | Wholesale & retail | 2023 |
| Olif AS | | 2025 |
| Olio Food Truck AS | Hospitality | 2023 |
| Ollbjørg AS | | 2021 |
| Olsøy AS | Health & social care | 2015 |
| Oluffa AS | Professional & scientific | 2013 |
| Omar's City Renhold AS | Administrative & support | 2007 |
| Omax Consulting AS | Information & communication | 2025 |
| Omer Transport AS | Transport & logistics | 2017 |
| Omfang AS | Wholesale & retail | 2022 |
| Omml Forvaltning AS | | 2026 |
| Omnia Solutions AS | Wholesale & retail | 2012 |
| On Energi AS | Construction | 2012 |
| On the Record AS | Wholesale & retail | 2025 |
| Onar Botten Arkitektur AS | Professional & scientific | 2011 |
| Onboard Holding AS | Administrative & support | 2021 |
| Onboard Norway AS | Administrative & support | 2021 |
| One Hundred Euro Club Da | Arts & recreation | 2022 |
| One Norseman AS | Manufacturing | 2021 |
| One Norseman Proshop AS | Wholesale & retail | 2025 |
| Onelda AS | Wholesale & retail | 2023 |
| Onesettle AS | Financial services | 2016 |
| Onkel Henrys Tattoo Shop Trondheim AS | Other services | 2014 |
| Onkel Svanhild AS | Manufacturing | 2019 |
| Onm Invest AS | Professional & scientific | 2021 |
| Onner Investment AS | | 2016 |
| Onpoint Intelligence AS | Information & communication | 2022 |
| Onpoint Intelligence LLC Nuf | Professional & scientific | 2022 |
| Ons Konsulenttjenester AS | Professional & scientific | 2003 |
| Ontario AS | Financial services | 2008 |
| Onv 49 AS | Real estate | 2022 |
| Onv 49 Og 51 AS | Real estate | 2019 |
| Onwork AS | Information & communication | 2020 |
| Oore Transport Drift AS | | 2023 |
| Opak Trondheim AS | Professional & scientific | 2018 |