| Oulun Sähködiesel Oy | Wholesale & retail | 1981 |
| Oulun Sähköspotti Oy | Construction | 2012 |
| Oulun Sähkötek Oy | Construction | 1994 |
| Oulun Sähköurakointi Oy | Construction | 2021 |
| Oulun TAK-Holding Oy | Professional & scientific | 2023 |
| Oulun TAK-Kiinteistöt Oy | Real estate | 1980 |
| Oulun TP Asennus ja Huolto Oy | Wholesale & retail | 2008 |
| Oulun Takka Oy | Construction | 2017 |
| Oulun Taksikuljetus Oy | Transport & logistics | 1998 |
| Oulun Taksimatkat Avoin Yhtiö | Transport & logistics | 1994 |
| Oulun Taloanalyysi Oy | Real estate | 2013 |
| Oulun Taloremontti Oy | Real estate | 1993 |
| Oulun Taloste Ky | | 1991 |
| Oulun Techuolto Oy | Real estate | 2000 |
| Oulun Tehdastalo Oy | Real estate | 1952 |
| Oulun Teknoparkki Oy | Real estate | 1997 |
| Oulun Telescan Oy | Construction | 1992 |
| Oulun Telesähkö Oy | Manufacturing | 1996 |
| Oulun Teollisuusapu Oy | Manufacturing | 2022 |
| Oulun Termet Ky | Manufacturing | 1985 |
| Oulun Terminaali Oy | Real estate | 2006 |
| Oulun Tervatalot Oy | Real estate | 2007 |
| Oulun Terähuolto Oy | Professional & scientific | 2006 |
| Oulun Terästyö Ky | | 1990 |
| Oulun Testikuva L. Tiusanen Ky | Other services | 1983 |
| Oulun Tili- ja Yrityspiste Oy | Professional & scientific | 1988 |
| Oulun Tilitieto Oy | Professional & scientific | 2001 |
| Oulun Tilitoimi Oy | Professional & scientific | 1967 |
| Oulun Timanttiporaus Oy | Construction | 2022 |
| Oulun Timanttisaha Oy | Construction | 1980 |
| Oulun Tiptop Oy | Administrative & support | 2023 |
| Oulun Toimistogalleria Oy | Real estate | 1986 |
| Oulun Toimitila Oy | Real estate | 2017 |
| Oulun Tuotantokeittiö Oy | Manufacturing | 2017 |
| Oulun Tuotekuja 1 Kiinteistöosakeyhtiö | Real estate | 2008 |
| Oulun Turbo- ja Autohuolto Oy | Other services | 1985 |
| Oulun Työkonepalvelu Oy | Real estate | 1954 |
| Oulun Työstökeskus Oy | Manufacturing | 1996 |
| Oulun Unikulma Oy | Wholesale & retail | 2006 |
| Oulun Uusihammas Oy | Manufacturing | 2015 |
| Oulun Vadelmaiset oy | Wholesale & retail | 2015 |
| Oulun Vaihtoautot Oy | Wholesale & retail | 2017 |
| Oulun Valhalla Oy | Real estate | 2018 |
| Oulun Valjasparkki Oy | Transport & logistics | 2019 |
| Oulun Vaneri Oy | Wholesale & retail | 2022 |
| Oulun Vesieristys Oy | Construction | 1997 |
| Oulun Viherpalvelu Ky | Professional & scientific | 2011 |
| Oulun Vihertalo Ky | Agriculture & fishing | 1996 |
| Oulun Viihdemixeri Ky | Professional & scientific | 2000 |
| Oulun Villa-Metsola Oy | Real estate | 1996 |
| Oulun Voudintie Oy | Real estate | 2015 |
| Oulun Wellhottaret Oy | Administrative & support | 1997 |
| Oulun Ydinkeskustan Parkki Oy | Transport & logistics | 2005 |
| Oulun Yhtiörakentajat Oy | Administrative & support | 1989 |
| Oulun Ykkösrakennus Oy | Construction | 2005 |
| Oulun Yksityinen Keuhkoklinikka Oy | Health & social care | 2002 |
| Oulun Yleisvaraosa ja Kone Ky | Wholesale & retail | 1983 |
| Oulun Yrityskalusto Oy | Wholesale & retail | 2006 |
| Oulun ammattipalvelu Oy | Wholesale & retail | 1994 |
| Oulun auto- ja rengashuolto oy | Other services | 2021 |
| Oulun autohuolto ja diagnostiikka Oy | Other services | 2025 |
| Oulun fyysikkopalvelut Oy | Professional & scientific | 2022 |
| Oulun hitsaus ja putkiasennus Oy | Construction | 2015 |
| Oulun kaivuripalvelu Oy | Construction | 2013 |
| Oulun kalustekeskus Ky | | 1989 |
| Oulun kelkkavuokraus Oy | Administrative & support | 2023 |
| Oulun kiinteistötyö Oy | Administrative & support | 2021 |
| Oulun kotipalvelut Oy | Other services | 2012 |
| Oulun kuntotarkastus Oy | Professional & scientific | 2022 |
| Oulun rakennuspalvelu oy | Construction | 2025 |
| Oulun ruokapalvelut Oy | Hospitality | 2015 |
| Oulun seudun Kiinteistöpalvelut Oy | Real estate | 2006 |
| Oulun seudun maalaus Oy | Construction | 2012 |
| Oulun Åströmin Parkki Oy | Transport & logistics | 2015 |
| Oulunmarja Oy | Hospitality | 2022 |
| Oulunsalo Pilot Holding Ky | Real estate | 2012 |
| Oulunseudun Fysioterapia Oy | Health & social care | 2008 |
| Oulunseudun Hoiva Oy | Health & social care | 2020 |
| Oulusa Oy | Administrative & support | 2026 |
| Oulux Ovet ja Ikkunat Oy | Manufacturing | 2018 |
| Ounaa Itsesi avoin yhtiö | Wholesale & retail | 2023 |
| Ounas Health Oy | Information & communication | 2025 |
| Ounas Productions Oy | Arts & recreation | 2024 |
| Oura Health Oy | Professional & scientific | 2013 |
| Ouratek Oy | Construction | 2006 |
| Outdoor Lahdenperä Oy | Administrative & support | 2024 |
| Outdoors Oulu Oy | Administrative & support | 2017 |
| Outel Oy | Manufacturing | 1978 |
| Outex Ky | Wholesale & retail | 1992 |
| OutiZek Ky | Wholesale & retail | 2017 |
| Outin Pirssit Oy | Transport & logistics | 2018 |
| Outstoke Holding Oy | Financial services | 2015 |
| Outstoke Oy | Information & communication | 2013 |
| Over Pool Oy | Financial services | 2025 |
| OverWell Oy | Other services | 2019 |
| Overcast Oy | Financial services | 2016 |
| OviCo Oy | Wholesale & retail | 2012 |
| Ovidian asunnot Oy | Real estate | 2020 |
| Owatec Group Oy | Professional & scientific | 2013 |
| Owatec Service Oy | Manufacturing | 2014 |