| OKA-Metsäpalvelu Oy | Wholesale & retail | 2004 |
| OMG Ohjelmapalvelut Oy | Arts & recreation | 2024 |
| ONNEN TÄHTEET osuuskunta | Real estate | 2000 |
| Osuuskunta Naava | Water & waste | 2002 |
| OVAK INVEST Oy | Financial services | 2018 |
| Obscure Sounds Oy | Arts & recreation | 2025 |
| Ohto Group Oy | Real estate | 2020 |
| Oikarinen Yhtiöt Oy | Transport & logistics | 2025 |
| Oil House Finland Oy | Manufacturing | 2012 |
| Ojavuon Taimitarha Oy | Agriculture & fishing | 2004 |
| Oksalämpö Oy | Energy | 1971 |
| Omply Finland Oy | Health & social care | 2025 |
| Omply Hardware Oy | Health & social care | 2025 |
| Omply Health Oy | Information & communication | 2025 |
| OnCon Oy | Professional & scientific | 2016 |
| Onnen versot Ky | Professional & scientific | 2011 |
| Oonan marja Oy | Wholesale & retail | 2021 |
| Open Desk Oy | Information & communication | 2022 |
| Open Health AVO Oy | Professional & scientific | 2014 |
| Opinkontti Keränen & Sirviö | Health & social care | 1998 |
| Optikko Kaakinen Oy | Wholesale & retail | 2013 |
| Optimisuunnittelu Kommandiittiyhtiö | Professional & scientific | 2005 |
| Optirad oy | Health & social care | 2002 |
| Orto-Wish Oy | Health & social care | 2007 |
| Osuuskauppa Maakunta | Wholesale & retail | 1917 |
| Osuuskunta Hankipuisto | Water & waste | 2002 |
| Osuuskunta Huuhkaja | Water & waste | 2003 |
| Osuuskunta Kainuun Kanerva | Administrative & support | 1995 |
| Osuuskunta Kulunnan Vesi | Construction | 2005 |
| Osuuskunta YH Raatosuo | Water & waste | 2003 |
| Otanmäen Lämpö Oy | Energy | 1998 |
| Oulujärven Laiva-Osakeyhtiö | Transport & logistics | 2011 |
| Oulujärven Metsäpalvelu Oy | Agriculture & fishing | 2022 |
| Ounel Energiapalvelu Oy | Wholesale & retail | 2016 |
| Outrik Oy | Other services | 2013 |
| Oy Angestar Ltd | Wholesale & retail | 2003 |
| Oy Cimkey Ltd | Manufacturing | 1991 |
| Oy Peenu & Co. Ab | Administrative & support | 2018 |
| Oy Pesonen Invest Ltd. | Information & communication | 2025 |
| Oy Suomen Terassiyhtiö Ab | Wholesale & retail | 2019 |
| Oy Waativa Ab | Professional & scientific | 2025 |
| P & P Väyrynen Oy | Professional & scientific | 1986 |
| PH Korhonen Oy | Construction | 2021 |
| PIHA JUSSI Oy | Administrative & support | 2012 |
| PORAPEKKA Oy | Construction | 2019 |
| POWCO Oy | Information & communication | 2024 |
| PP-Timantti Oy | Construction | 2024 |
| PR & Partners Oy | Professional & scientific | 2023 |
| Profit Technology Avoinyhtiö | Manufacturing | 2016 |
| PRT Oulujärvi Oy | Transport & logistics | 2018 |
| PSM Rakennepalvelu Oy | Professional & scientific | 2013 |
| PTA Palvelut Oy | Construction | 2016 |
| Paavo Enroth Oy | Professional & scientific | 2010 |
| Padelman Oy | Arts & recreation | 2021 |
| Paikallis-Sähkö Oy | Construction | 1963 |
| Palatikki kommandiittiyhtiö | | 1981 |
| Palmosa Ky | | 1988 |
| Paltaniemen vesi- ja viemäriosuuskunta | Water & waste | 1972 |
| Panda Pizza & Kebab Kajaani Oy | Hospitality | 2026 |
| Pankka Group Oy | Professional & scientific | 2023 |
| Parkinniemen Puutarha Oy | Agriculture & fishing | 2006 |
| Parturi Antalia Oy | Other services | 2022 |
| Parturi Burhan Oy | Other services | 2024 |
| Parturi Istanbul Oy | Other services | 2019 |
| Parturi-Kampaamo Mia Komulainen Ky | Other services | 2002 |
| Parturi-kampaamo Oasis Oy | Other services | 2009 |
| Pasi Suutari Oy | Agriculture & fishing | 2018 |
| Patkos Oy | Real estate | 2017 |
| Pattijoen Kullero Ky | Wholesale & retail | 1993 |
| Paumer Oy | Construction | 1996 |
| Pausella Games Oy | Information & communication | 2023 |
| PaySki Oy | Information & communication | 2026 |
| Pekka Agarth Oy | Information & communication | 2020 |
| Pekka Heikkinen & kumpp. Oy | Wholesale & retail | 2009 |
| Pekka Ovaska osakeyhtiö | Professional & scientific | 1996 |
| Pelikasvatus Kainuu Oy | Education | 2023 |
| Peltep Oy | Real estate | 2017 |
| Perhe Sirviö Osakeyhtiö | Professional & scientific | 2008 |
| Petroviz oy | Transport & logistics | 2020 |
| Peuraniemen Taimitarha Oy | Agriculture & fishing | 1996 |
| Pharma Safety Oy | Wholesale & retail | 2023 |
| Phocula Oy | Health & social care | 1989 |
| Piiraisen Viherpalvelu Oy | Administrative & support | 1988 |
| Piksa Oy | Professional & scientific | 1986 |
| Pirkkari Oy | Manufacturing | 2011 |
| Pizzeria Alanya Oy | Hospitality | 2023 |
| Pizzeria Casabianco Ky | | 1981 |
| Pizzeria Pikkukettu Oy | Hospitality | 2021 |
| Planray Oy | Construction | 1975 |
| Pohjois-Suomen Kuivaustekniikka Oy | Professional & scientific | 1995 |
| Pohjois-Suomen Monipalvelu Oy | Transport & logistics | 2022 |
| Pohjois-Suomen Rakennuslogistiikka Oy | Construction | 2020 |
| Pohjoisen Hamppu Oy | Wholesale & retail | 2022 |
| Pohjolan Ekolämpö Oy | Construction | 2009 |
| Pohjolan Euro-Kate Oy | Construction | 1992 |
| Pohjolan Kaivu Ky | Construction | 2009 |
| Pohjolan Kylmätekniikka Oy | Manufacturing | 2012 |
| Pohjolan Pesukarhu Oy | Other services | 2013 |
| Pohjolan Tavarakirppis Oy | Wholesale & retail | 2008 |
| Pohrama Oy | Construction | 1991 |