| Niitynlaidan Oy | Administrative & support | 2011 |
| Nipetek Oy | Construction | 2023 |
| Nipsinki Oy | Real estate | 2019 |
| Nispet Oy | Financial services | 2024 |
| Nissinen Yhtiöt Oy | Professional & scientific | 2024 |
| Nollasta viiteen Oy | Professional & scientific | 2023 |
| NordCleanLaser Oy | Manufacturing | 2026 |
| Nordic Street Lights Oy | Wholesale & retail | 2015 |
| NordicHost Oy | Information & communication | 2024 |
| Norre Software Oy | Information & communication | 2020 |
| Norre Työterveys Oy | Health & social care | 2023 |
| Nosto ja kuljetus Matti Salin Oy | Transport & logistics | 2008 |
| Nosturiliike Sulkala Oy | Construction | 1982 |
| Nuohous Kipinä Oy | Construction | 2019 |
| OBL-invest Oy | Real estate | 2019 |
| OK Aurinkovoima Oy | Energy | 2022 |
| OK Management Consulting Oy | Professional & scientific | 2025 |
| OK Sähkövarasto Oy | Energy | 2026 |
| OKP Kuljetus Oy | Transport & logistics | 2003 |
| Osk Oulupro | Professional & scientific | 2020 |
| OSp Oulun Saneerauspalvelu Ky | Construction | 2000 |
| OTR Tekno-Rakennus Oy | Construction | 1990 |
| Odonto Investments Oy | Health & social care | 2025 |
| Ojan Rauta Oy | Wholesale & retail | 1964 |
| Olkaa Oy | Professional & scientific | 2022 |
| Ollila Coaching and Real Estate Oy | Real estate | 2024 |
| OloWorks Oy | Arts & recreation | 2020 |
| Omabox Oy | Hospitality | 2023 |
| Omapaja Holding Oy | Information & communication | 2016 |
| Omapaja Oy | Education | 2011 |
| Omapaja Talouspalvelut Oy | Administrative & support | 2018 |
| Omapaja Valmennus Oy | Administrative & support | 2018 |
| Oneus Oy | Professional & scientific | 2021 |
| Oovee Oy | Construction | 2024 |
| Optimum Invest Oy | Real estate | 2021 |
| Oripää Oripäänkangas Tuulivoima Oy | Professional & scientific | 2013 |
| Ossi Agency & Holdings Oy | Wholesale & retail | 2025 |
| Ostatarra Finland Oy | Manufacturing | 2008 |
| Otopro Oy | Administrative & support | 2011 |
| OuSem Oy | Information & communication | 2014 |
| Oukone Oy | Real estate | 2021 |
| Oulun Ase Oy | Wholesale & retail | 2018 |
| Oulun Auto- ja Korimaalaus Ky | Other services | 1990 |
| Oulun Automaatiopalvelu Oy | Wholesale & retail | 1995 |
| Oulun B&N Yhtiöt Oy | Professional & scientific | 2008 |
| Oulun DM-Auto Oy | Wholesale & retail | 1994 |
| Oulun Fitness-palvelut Oy | Arts & recreation | 2008 |
| Oulun Haaransuontie 6 Kiinteistö Oy | Real estate | 2013 |
| Oulun Haaransuontie 8 Kiinteistö Oy | Real estate | 2006 |
| Oulun Keittiöidea Oy | Wholesale & retail | 2001 |
| Oulun Köökki Oy | Wholesale & retail | 2004 |
| Oulun Laatuasennus Oy | Construction | 2008 |
| Oulun Lasikeisarit Oy | Construction | 2022 |
| Oulun Liikenne Oy | Transport & logistics | 2006 |
| Oulun Loistosähkö Oy | Construction | 2006 |
| Oulun Matkatoimisto Oy | Administrative & support | 2009 |
| Oulun OK Palvelut Oy | Construction | 2017 |
| Oulun Piharakennus Oy | Construction | 1996 |
| Oulun Rakennusinssi Group Oy | Construction | 2020 |
| Oulun Romu Oy | Wholesale & retail | 2013 |
| Oulun Seudun Näkökulma Oy | Wholesale & retail | 2025 |
| Oulun Seudun Sähkö Kuitu Oy | Information & communication | 2017 |
| Oulun Seudun Sähkö Verkkopalvelut Oy | Energy | 1994 |
| Oulun Säiliökuljetus Oy | Transport & logistics | 1971 |
| Oulun Teollisuuskojeistot Oy | Manufacturing | 1995 |
| Oulun Traktori- ja Konehuolto Kontiokari Oy | Manufacturing | 1991 |
| Oulun Työstökonehuolto Oy | Manufacturing | 2013 |
| Oulun puutarha apu oy | Transport & logistics | 2026 |
| Oulunsalon Asu-Piste Oy | Wholesale & retail | 2005 |
| Oulunsalon Auto ja Konevälitys Ky | Wholesale & retail | 1989 |
| Oulunseudun Autohuolto Oy | Other services | 2020 |
| Ouplan Oy | Professional & scientific | 2021 |
| Outa Invest Oy | Financial services | 2010 |
| Ovica Oy | Administrative & support | 2022 |
| Oy Agellus Ab | Wholesale & retail | 1996 |
| Oy Erkki Juurus Ltd. | Wholesale & retail | 2024 |
| Oy Kala-Fisk J.Dahlström Ab | Agriculture & fishing | 2015 |
| Oy Luminosity Ab | Professional & scientific | 2017 |
| Oy Ylivoimala Ab | Other services | 2010 |
| P. Alanko Oy | Professional & scientific | 2021 |
| P.S. Ervasti Oy | Transport & logistics | 2013 |
| PCS-Engineering Oy | Professional & scientific | 2004 |
| PH Capital Oy | Professional & scientific | 2020 |
| PHG Holding Oy | Financial services | 2025 |
| PIE Holding Oy | Professional & scientific | 2026 |
| PJ-Tekniikka Oy | Professional & scientific | 2005 |
| PJK Holding Oy | Real estate | 2024 |
| PKC Group Oy | Manufacturing | 1994 |
| PKC Wiring Systems Oy | Wholesale & retail | 1991 |
| PML Capital Oy | Wholesale & retail | 2024 |
| PP-Traktori Oy | Wholesale & retail | 2016 |
| PV-Process Valmennus Oy | Education | 1999 |
| Paakkari Solutions Oy | Professional & scientific | 2026 |
| Paateli Oy | Wholesale & retail | 2020 |
| Padel Liminka Oy | Arts & recreation | 2021 |
| Padel Pohjoinen Oy | Arts & recreation | 2021 |
| Padel Ylivieska Oy | Arts & recreation | 2021 |
| Paiturin Kiinteistö Oy | Real estate | 2018 |
| Pajala Pohjois-Suomi Oy | Construction | 2020 |
| Pajala-HVM Housing Oy | Real estate | 2025 |