| Thermocold KFD AS | Manufacturing | 1991 |
| Thinthan Asia Food AS | Wholesale & retail | 2019 |
| Tiger Securities AS | | 2023 |
| Tinnlageret AS | | 2021 |
| TK Strøm AS | Construction | 2013 |
| TK Utrykning AS | Education | 2020 |
| TK Veod Oü | Transport & logistics | 2017 |
| Tmcm AS | Financial services | 2015 |
| TN Autopartner AS | Wholesale & retail | 2015 |
| To - Tum AS | Manufacturing | 2005 |
| Todo Byggservice AS | Construction | 2018 |
| Topp Fritid AS | Wholesale & retail | 2004 |
| Torma AS | Health & social care | 2024 |
| Torpebaugen 4 AS | Real estate | 2018 |
| Torvlia Invest AS | Real estate | 1997 |
| Tro Eiendom AS | Real estate | 2012 |
| Trygg Bane AS | Professional & scientific | 2022 |
| TSS Eiendom AS | Transport & logistics | 2024 |
| Tång Rør & Sveiseservice AS | Construction | 2025 |
| Tømmerholt Armering AS | Construction | 2019 |
| Tømmerholt Eiendom AS | Real estate | 2019 |
| Tømmerholt Invest AS | Real estate | 1994 |
| Unike Reiser AS | Administrative & support | 2022 |
| Unison Holding AS | | 2026 |
| Utgårdskilen Utvikling AS | Real estate | 2023 |
| Utne Holding AS | Transport & logistics | 2005 |
| Utne Transport AS | Transport & logistics | 1973 |
| Utvendig Renhold Østfold AS | Administrative & support | 2025 |
| Valø Invest AS | Professional & scientific | 2011 |
| Vaske Og Transporttjenester AS | Administrative & support | 2021 |
| Vatvedt AS | Real estate | 1989 |
| Vatvedt Holding AS | Professional & scientific | 2023 |
| Vatvedt Industri AS | Manufacturing | 2007 |
| Vatvedt Technology AS | Professional & scientific | 1994 |
| Verkstedgrossisten Sarpsborg AS | Wholesale & retail | 2016 |
| Vestre Grimsøy Vann- Og Avløpslag SA | Water & waste | 2008 |
| Viken Heis AS | Construction | 2021 |
| Viken Heisalarm AS | Administrative & support | 2024 |
| Viken Security AS | Construction | 2020 |
| Viking Eiendomsutvikling AS | Construction | 2017 |
| Vision Security-Systems AS | Construction | 2009 |
| Vivax AS | Professional & scientific | 2024 |
| VJ Transport AS | Transport & logistics | 2017 |
| Vra Holding AS | | 2021 |
| Wallin Holding AS | | 2008 |
| Wethunes Holding AS | | 2024 |
| Wick Investment AS | Financial services | 2013 |
| Wics Holding AS | Real estate | 2025 |
| Winmar AS | | 2025 |
| X3 Consult AS | Professional & scientific | 2021 |
| Ynwa-Eiendom AS | Real estate | 1987 |
| Yvo Holding AS | | 2025 |
| Øketangen SA | Water & waste | 2009 |
| Ørebekk SA | Water & waste | 2008 |
| Østfold Avlastningssenter Da | Health & social care | 2009 |
| Østfold Bildemontering Eiendom AS | Wholesale & retail | 1984 |
| Østfold Boligutvikling AS | Real estate | 2018 |
| Østfold Gass & VVS AS | Construction | 2008 |
| Østfold Hvitevaremontering AS | Construction | 2016 |
| Østfold Kran & Transport AS | Transport & logistics | 2005 |
| Østfold Pizza Iseveien AS | Hospitality | 2017 |
| Østfold Ventilasjonsrens AS | Administrative & support | 2026 |
| Østlandske Inneklima AS | Professional & scientific | 2014 |