| Yasmine | Wholesale & retail | 1987 |
| Yggdrasil Livets Tre AS | Wholesale & retail | 2024 |
| Yko AS | Education | 2024 |
| Yogy AS | Arts & recreation | 2017 |
| Yuen Eiendom AS | Real estate | 2024 |
| Yuen Holding AS | Real estate | 2022 |
| Zaklad Uslugowy Monter Radoslaw Ciba | Construction | 2005 |
| Zazzera Holding AS | | 2023 |
| Ze Bra Eiendom AS | Real estate | 2021 |
| Ze Bra Holding AS | Construction | 2018 |
| Ze Bra Vei AS | Construction | 1989 |
| Zenon Rådgivning AS | Professional & scientific | 2020 |
| Älvsbyhus Norge AS | Construction | 1995 |
| Ålum Solberg AS | Wholesale & retail | 2015 |
| Årefjordtangen 2 Drift SA | Water & waste | 2021 |
| Årefjordtangen Avløp Drift SA | Water & waste | 2018 |
| Årefjordtangen Avløp SA | Real estate | 2016 |
| Årvoll Konferansesenter AS | Real estate | 2022 |
| Årvoll Næringseiendom AS | Real estate | 2006 |
| Årvollskogen 10 AS | Real estate | 2025 |
| Årvollskogen 25 AS | Real estate | 2022 |
| Årvollskogen 32 AS | Real estate | 2009 |
| Årvollskogen 79 AS | Real estate | 1998 |
| Årvollskogen 81 AS | Manufacturing | 2008 |
| Årvollskogen 90 AS | Real estate | 2024 |
| Årvollskogen 91 AS | Real estate | 1987 |
| Årvollskogen 95 AS | Real estate | 2007 |
| Årvollskogen 96 Eiendom AS | Real estate | 2013 |
| Åslia Holding AS | | 2021 |
| Åsveien 8 AS | Real estate | 2014 |
| Åsveien Invest AS | Real estate | 2005 |
| Øbas Da | Construction | 1990 |
| Øbu Prosjekt AS | Real estate | 2009 |
| Ødda Takst Da | Real estate | 2002 |
| Øf Holding AS | Construction | 2021 |
| Øisteins Gate 1 AS | Real estate | 2025 |
| Øisteins Gate 10 AS | Real estate | 2020 |
| Øisteins Gate 12 AS | Real estate | 2020 |
| Øk Invest AS | Professional & scientific | 2022 |
| Øk Teknikk AS | Professional & scientific | 2022 |
| Økonomi Forum AS | Real estate | 1992 |
| Øksnes CNC AS | Manufacturing | 2023 |
| Øksnes Holding AS | | 2023 |
| Øksnes Teknikk AS | Professional & scientific | 2023 |
| Ømf Østfold AS | Construction | 2022 |
| Øre Villa AS | Hospitality | 2014 |
| Ørenesehals Klinikken AS | Health & social care | 2016 |
| Øreveien 11B AS | Real estate | 1978 |
| Øsi Allservice AS | Manufacturing | 2000 |
| Øst Eiendom ST. Mariesgate 42 AS | Real estate | 2009 |
| Øst Innkvartering AS | Real estate | 2019 |
| Øst Skogpleie AS | Agriculture & fishing | 2022 |
| Østavind Arkitekter AS | Professional & scientific | 1995 |
| Østbergs AS | Real estate | 2005 |
| Østbergs Eiendom AS | Real estate | 2018 |
| Østensjø Økologiske AS | Professional & scientific | 2014 |
| Østfold Eiendom 1 AS | | 2024 |
| Østfold Eiendom 3 AS | Construction | 2024 |
| Østfold Follo Nyskapingsfond AS | Financial services | 2016 |
| Østfold Møbler AS | Wholesale & retail | 2008 |
| Østfold Plastikkirurgi AS | Health & social care | 2013 |
| Østfold Sports Holding AS | Wholesale & retail | 2007 |
| Østfold Varmeservice Øvas AS | Construction | 1998 |
| Østland Bygg & Bil AS | Construction | 2024 |
| Østlandske Eiendomsdrift AS | Real estate | 2012 |
| Østlandske Hjertesenter AS | Health & social care | 2016 |
| Øvereng Interiør AS | Wholesale & retail | 1998 |
| Øyelege LH Berger AS | Health & social care | 2014 |
| Øyelegene I Moss AS | Health & social care | 2021 |
| Øymoe Holding AS | | 2024 |
| Øyverket AS | | 2026 |