| OR - RK - rakennus oy | Construction | 1994 |
| OT-Con Oy | Construction | 2010 |
| OY Steel & Artworks Nordic LTD | Manufacturing | 2018 |
| Ocab Finland Holding Oy | Construction | 2024 |
| Oculux Oy | Administrative & support | 2000 |
| Office for Peripheral Architecture Oy | Professional & scientific | 2002 |
| Offset-Tekari Oy | Manufacturing | 1986 |
| Oheisvire Oy | Education | 2018 |
| Ohjelmatoimisto StickOut Kommandiittiyhtiö | Professional & scientific | 1998 |
| Ohjelmistokonsultointi Kohtala Oy | Information & communication | 2018 |
| Oiva Isännöinti Seinäjoki Oy | Financial services | 2016 |
| OivaOvi Oy | Construction | 2025 |
| Ojala Motors Oy | Other services | 2021 |
| Olavi Kivistö Oy | Other services | 1986 |
| Old Grill Pub Oy | Hospitality | 1988 |
| Oliveira Oy | Hospitality | 1989 |
| Oliver ja Kumppanit Oy | Hospitality | 2007 |
| OmaMappi Oy | Professional & scientific | 2022 |
| Omia Oy | Professional & scientific | 2009 |
| OnGluon Oy | Professional & scientific | 2024 |
| OnePomp Oy | Wholesale & retail | 2023 |
| Onnensilmä Oy | Health & social care | 2006 |
| Oona Huhtamäki Oy | Financial services | 2022 |
| OpSec Oy | Administrative & support | 2009 |
| Operoiva Oy | Professional & scientific | 2024 |
| Opti-Design Oy | Wholesale & retail | 2019 |
| Optimic Ky | Wholesale & retail | 2012 |
| Optimum Fleet Oy | Administrative & support | 2022 |
| Orcator Oy | Professional & scientific | 2025 |
| Oree Holding Oy | Professional & scientific | 2024 |
| Oree Oy | Manufacturing | 2017 |
| Orrenmaa Holding Oy | Financial services | 2017 |
| OrtoKukk Oy | Health & social care | 2011 |
| Ortoserv Oy | Wholesale & retail | 2002 |
| Ortospede Oy | Health & social care | 2022 |
| Orvokki Investment Oy | Professional & scientific | 2024 |
| Osaweb Oy | Wholesale & retail | 2021 |
| Oskarinkadun Kiinteistöt Oy | Real estate | 1950 |
| Osmo Mäenpää Oy | Wholesale & retail | 2000 |
| Ospemet Oy | Manufacturing | 2014 |
| Ostofokus Oy | Professional & scientific | 2017 |
| Osuuskunta Nostates | Arts & recreation | 2025 |
| Osuuskunta Vittingin Kärrynpyörä | Real estate | 1997 |
| Osuva Isännöinti Oy | Real estate | 2025 |
| Otus-Tuotanto Oy | Arts & recreation | 2007 |
| Ovea Ky | Manufacturing | 1986 |
| OverLine ENT Oy | Arts & recreation | 2025 |
| Ovimestariasennus Oy | Construction | 2020 |
| Owl Capital Oy | Financial services | 2026 |
| Oy Alma Group Hotels Ab | Hospitality | 2001 |
| Oy Apex Hammasklinikka | Real estate | 1992 |
| Oy Auto - Way Ltd | Wholesale & retail | 2012 |
| Oy BBS-Plan Ltd | Wholesale & retail | 2004 |
| Oy BITIM Business Innovation & Technology Management Inter national Ltd | Real estate | 1993 |
| Oy BioSorbio Ltd | Manufacturing | 2018 |
| Oy Cash-Man Information Ltd | Professional & scientific | 1990 |
| Oy Complete Music RH Ltd | Information & communication | 2011 |
| Oy Diamond King Ab Ltd | Information & communication | 2011 |
| Oy Eastcon-Finland Ltd | Administrative & support | 1993 |
| Oy Eurofire Ab | Wholesale & retail | 2008 |
| Oy Finnagria Ab | Wholesale & retail | 2021 |
| Oy Finnladek F-L Ab | Real estate | 1990 |
| Oy Fokusvia Ab | Hospitality | 1994 |
| Oy HM Profiili Ab | Wholesale & retail | 2002 |
| Oy Handel Henkilöstöpalvelut Ab | Administrative & support | 2024 |
| Oy Happy Valley Tractor Ltd | Wholesale & retail | 1989 |
| Oy Hepatic Home Invest Ltd | Financial services | 1991 |
| Oy Hobbyfinn Global Ltd | Real estate | 1993 |
| Oy Hopotihoi Suomen Lelukamarit Toy & Hobby Ltd | Wholesale & retail | 1994 |
| Oy Interlux Ab | Professional & scientific | 1992 |
| Oy JJ Investori Ltd | Real estate | 2011 |
| Oy KH-Morrit Ltd | Wholesale & retail | 1999 |
| Oy Kangas-International Ltd | Professional & scientific | 1997 |
| Oy Kaskisten Energia-Kaskö Energi Ab | Energy | 2004 |
| Oy Mariet Plast Ltd | Manufacturing | 1999 |
| Oy Ment Ab | Wholesale & retail | 1984 |
| Oy Nordic 7 Ltd | Wholesale & retail | 2019 |
| Oy Onnen Juurella | Hospitality | 2012 |
| Oy Operative Recovery Solutions JMR Ltd | Water & waste | 2011 |
| Oy Pekicomp Ltd | Real estate | 1996 |
| Oy Protective JJL Mediwear Ltd | Wholesale & retail | 2020 |
| Oy RST's Ocean-Trade Finland Ltd | Wholesale & retail | 1995 |
| Oy Raffinato Ab | Professional & scientific | 1991 |
| Oy Really Pronto Ltd | Wholesale & retail | 1996 |
| Oy Rhonda Services Ab | Professional & scientific | 2004 |
| Oy Rikory Ab | Professional & scientific | 1996 |
| Oy SK-Reviisorit Ab | Professional & scientific | 2009 |
| Oy SNE Environment Engineering Ltd | Professional & scientific | 1992 |
| Oy Scandinavian Show House Ltd | Professional & scientific | 1995 |
| Oy Strand Kapital Ab | Professional & scientific | 2015 |
| Oy Success Key Ltd | Wholesale & retail | 1996 |
| Oy Swankrona Ab | Professional & scientific | 1994 |
| Oy Sze Titan Exports Ltd | Wholesale & retail | 1993 |
| Oy Travelnet Ltd | Professional & scientific | 1996 |
| Oy West Oasis Ab | Wholesale & retail | 2022 |
| P & K Kodit Oy | Real estate | 2025 |
| P&R Group Oy | Information & communication | 2021 |
| P-Productions Osakeyhtiö | Manufacturing | 2024 |
| PA Investo Oy | Real estate | 2023 |
| PA- Motor Service Oy | Other services | 2025 |