| Olkkolan vesiosuuskunta | Water & waste | 1998 |
| OmaMestari Oy | Information & communication | 2022 |
| Opaja Consulting Oy | Professional & scientific | 2023 |
| Opaja Oy | Professional & scientific | 2013 |
| Oplax Mikkeli Oy | Manufacturing | 2019 |
| Optiikka Salmela Oy | Wholesale & retail | 2010 |
| Optikko Raimo Laitinen Oy | Wholesale & retail | 1996 |
| Oranen Consulting Oy | Professional & scientific | 2020 |
| Ortomix Oy | Health & social care | 2013 |
| Ossin aarrearkku Oy | Financial services | 2022 |
| Ostaranova Oy | Professional & scientific | 2012 |
| Osuuskauppa Suur-Savo | Wholesale & retail | 1916 |
| Osuuskunta Metio | Administrative & support | 2017 |
| Osuuskunta Yhteystila | Real estate | 2026 |
| Otavan Louhinta Ky | Construction | 1992 |
| Otavan Maalaustyö Oy | Construction | 2004 |
| Otavan OK-Rakennus Oy | Construction | 2017 |
| Oujee Family Oy | Hospitality | 2021 |
| Oy KJM-Trading Ltd | Construction | 1996 |
| Oy Limited Datacommunication MLI | Information & communication | 1999 |
| Oy Meditop Ltd | Health & social care | 1989 |
| Oy Oilkase Ab | Wholesale & retail | 1985 |
| Oy SWM-Wood Ltd | Manufacturing | 1998 |
| P & T Salmela Oy | Manufacturing | 2003 |
| P&T Rakennuspalvelu Oy | Construction | 2021 |
| P-Tuuma Oy | Construction | 2016 |
| PATE Investments Oy | Financial services | 2023 |
| PETS Holding Oy | Wholesale & retail | 2019 |
| PHM Finesse Oy | Health & social care | 2020 |
| PL-Kilpi Lehtonen Oy | Manufacturing | 2005 |
| PLC-Tec Oy | Manufacturing | 2010 |
| PLE Consulting Oy | Professional & scientific | 2019 |
| PM-Materak Oy | Construction | 1981 |
| Pokmi OY | Real estate | 2018 |
| POP! Taksit Oy | Transport & logistics | 2007 |
| POWERFUL AF Oy | Education | 2020 |
| PP Market Oy | Wholesale & retail | 2019 |
| PRV Asennus Oy | Manufacturing | 2024 |
| PRV Fitting Oy | Manufacturing | 1995 |
| PUCO Etelä-Suomi Oy | Construction | 2020 |
| PUCO Group Oy | Construction | 2018 |
| PUCO Itä-Suomi Oy | Construction | 2020 |
| PaPet Invest Oy | Real estate | 2025 |
| Paimport oy | Wholesale & retail | 2019 |
| Painotalo Printta Kommandiittiyhtiö | Manufacturing | 1992 |
| Palhomaa Oy | Real estate | 1998 |
| Pallot ilmassa JSKV OY | Education | 2025 |
| Palvelutrio Oy | Hospitality | 2004 |
| Palvelutukku Itä-Suomi Oy | Wholesale & retail | 1987 |
| Pamaus Oy | Manufacturing | 2024 |
| ParkFix Oy | Construction | 2021 |
| Parketti- ja Mattotyö M. Myyryläinen Oy | Construction | 1992 |
| Partti Consulting Oy | Professional & scientific | 2014 |
| Parturi-Kampaamo Duo Saksi Ky | Other services | 1991 |
| Parturi-Kampaamo Lei-Ra Oy | Other services | 2023 |
| Parturi-Kampaamo Niksi Ky | Other services | 1987 |
| Parturi-Kampaamo Salon Riley Leena Parviainen Ky | Other services | 1977 |
| Parturi-kampaamo In-Diko Oy | Other services | 2024 |
| Parturi-kampaamo Salon Pinkki Ky | Other services | 2010 |
| Parturiliike Red Oy | Other services | 1993 |
| Pasofix Mikkeli Oy | Manufacturing | 2021 |
| Patakatti Ky | Wholesale & retail | 2006 |
| Paula Makkonen Oy | Professional & scientific | 2023 |
| Pauliina Rundgrén HandiCrafts Oy | Arts & recreation | 2007 |
| Pekc Invest Oy | Wholesale & retail | 2022 |
| Pekka Korhonen Kiinteistöhallinto Ky | Real estate | 2012 |
| Pele Trading Oy | Hospitality | 2015 |
| PeliNikkarit Oy | Manufacturing | 2019 |
| Peltisepänliike Paavilainen Ky | Construction | 2009 |
| Peltisepänliike Pyyhtinen Ky | Construction | 1988 |
| Pelto Innovations Oy | Professional & scientific | 2024 |
| Perhekoti Elopelto Oy | Health & social care | 2001 |
| Perhekoti Jokipolku Oy | Health & social care | 2005 |
| Perhekoti Silta Ky | Health & social care | 2000 |
| Perhekoti Tähtipolku Oy | Health & social care | 2020 |
| Pertunmaan Lämpö Oy | Energy | 1995 |
| Petoja Oy | Construction | 2023 |
| Petsamo Consulting Oy | Professional & scientific | 2024 |
| Petsor Oy | Wholesale & retail | 2025 |
| Pienryhmäkoti Ankanpesä Oy | Health & social care | 2006 |
| Pihlaja Pub Ky | Hospitality | 1993 |
| Pihlajalinna Ikioma Oy | Health & social care | 2013 |
| Pillipiipari Oy | Administrative & support | 2023 |
| Pinky Inky Oy | Other services | 2024 |
| Pipemodul Oy | Manufacturing | 1999 |
| Pipo Labs Oy | Information & communication | 2017 |
| Pirunkeuhko Oy | Arts & recreation | 2019 |
| Pitkäjärven Auto-osat Oy | Wholesale & retail | 2005 |
| Pitkäjärven Eläinklinikka Oy | Professional & scientific | 2021 |
| Pito-Viinikukko Oy | Hospitality | 2018 |
| Planomet Oy | Wholesale & retail | 1995 |
| Platom Oy | Professional & scientific | 1998 |
| PlayHeaven Oy | Arts & recreation | 2025 |
| Pmpizzeria Max Oy | Hospitality | 2023 |
| Pohjalammen Ponnistus Oy | Hospitality | 2018 |
| Pohjolan Mustikka Oy | Wholesale & retail | 2015 |
| Pohjolan Villit Oy | Wholesale & retail | 2011 |
| PolarDeco Ky | Other services | 2017 |
| Polkupyörä Etappi Oy | Wholesale & retail | 2015 |
| Pomin Oy | Construction | 1995 |