| Oy Second Theatre Ltd | Information & communication | 2020 |
| Oy Secpro Ab | Wholesale & retail | 1991 |
| Oy Secure Life Finland Ltd | Financial services | 2000 |
| Oy Seed Tours Ltd | Administrative & support | 1996 |
| Oy Seege Ab | Wholesale & retail | 1973 |
| Oy Selk Ab | Financial services | 1976 |
| Oy Sendai Finland Ltd | Wholesale & retail | 1994 |
| Oy Septidin Ab | Manufacturing | 2022 |
| Oy Sergejeff Ab | Wholesale & retail | 1993 |
| Oy ServInEks Ab | Real estate | 2011 |
| Oy Sethonian Ab | Professional & scientific | 2013 |
| Oy Seweco Solglimt Ab | Financial services | 1994 |
| Oy Shadowland Ltd | Arts & recreation | 2003 |
| Oy Shampura Ltd | Financial services | 1999 |
| Oy Shipco Transport Ab | Transport & logistics | 2002 |
| Oy Shiptech Ab | Professional & scientific | 2010 |
| Oy Sifterium Ltd | Professional & scientific | 2002 |
| Oy Sigma Ab | Professional & scientific | 1953 |
| Oy Sigurds Gård Ab | Real estate | 2025 |
| Oy Sihaco Ab | Financial services | 1987 |
| Oy Siltasaarre Ab | Financial services | 1997 |
| Oy Silva Invest Ab | Professional & scientific | 1984 |
| Oy Silvacultura Ab | Agriculture & fishing | 1992 |
| Oy Simvest Ab | Financial services | 1984 |
| Oy Sineu Holding Ltd | Real estate | 2001 |
| Oy Sineu SA | Hospitality | 1997 |
| Oy Sirius Capital Partners Ab | Professional & scientific | 2014 |
| Oy Site Logic Ab | Professional & scientific | 2005 |
| Oy Sitehold Ab | Financial services | 1986 |
| Oy Siuntion Rantapuisto | Real estate | 2006 |
| Oy Skanno Ab | Wholesale & retail | 2000 |
| Oy Skeye Ab | Professional & scientific | 1990 |
| Oy Skinnvest Ab | Hospitality | 1987 |
| Oy Slurp Ab | Wholesale & retail | 2014 |
| Oy Sneckan Ab | Real estate | 2003 |
| Oy Solea Ab | Real estate | 2024 |
| Oy Solvit Ab | Professional & scientific | 2001 |
| Oy Sophie-International Ab | Professional & scientific | 1994 |
| Oy Soul-Productions Finland Ltd | Professional & scientific | 1997 |
| Oy Soveri Wildlife Films Ltd | Information & communication | 1991 |
| Oy Sovittelutoimisto TySo Ab | Education | 2016 |
| Oy Space nord-ship | Construction | 2022 |
| Oy Spandau Ab | Real estate | 2023 |
| Oy Spark2Boost Ab Ltd | Education | 2025 |
| Oy Speedster Corporation Ltd | Financial services | 1990 |
| Oy Spillway Invest Ab | Financial services | 2026 |
| Oy Spiritus Historiae Ab | Professional & scientific | 1989 |
| Oy Sports Car Center Ab | Wholesale & retail | 1992 |
| Oy Stadin Ratikat Ab | Wholesale & retail | 1991 |
| Oy Stahanov Ab | Professional & scientific | 1990 |
| Oy Stahlacier S.A. | Wholesale & retail | 1991 |
| Oy Stay Tuned Ltd | Information & communication | 1994 |
| Oy Stayhome Ab | Wholesale & retail | 2015 |
| Oy Stellarec Ltd | Arts & recreation | 2016 |
| Oy Stellaria Media Ab | Professional & scientific | 1997 |
| Oy Stentor Ab | Wholesale & retail | 1970 |
| Oy Stig-Mo Ab | Health & social care | 1991 |
| Oy Stimulator Ab | Professional & scientific | 1989 |
| Oy Stina Poutiainen Ab | Real estate | 2010 |
| Oy StoneAnt Ltd | Wholesale & retail | 2004 |
| Oy Stor-Mar Ltd | Financial services | 2020 |
| Oy Storholm Capital AB | Financial services | 2015 |
| Oy Strand & Arkipelag Ab | Real estate | 2014 |
| Oy Stratamax Ab | Professional & scientific | 1993 |
| Oy Streamasters Ltd. | Information & communication | 2016 |
| Oy Strongwell Ab | Professional & scientific | 1989 |
| Oy Studio Charlotta Boucht Ab | Professional & scientific | 1997 |
| Oy Studio L & B Ab | Professional & scientific | 2008 |
| Oy Styrbord Capital Ab | Financial services | 2015 |
| Oy Ståhlberg Consulting Ab | Health & social care | 2005 |
| Oy Sumel Ab | Real estate | 1961 |
| Oy Sundman Consulting Ab | Professional & scientific | 1962 |
| Oy Sundqvist Finland Ab | Wholesale & retail | 2009 |
| Oy Suomen Hilaris Ab | Professional & scientific | 1995 |
| Oy Suomen Jalokivilaboratorio Ab | Other services | 1984 |
| Oy Suomen Jyrä Ab | Professional & scientific | 2016 |
| Oy Suomen Jäätelö Ab | Wholesale & retail | 2015 |
| Oy Suomen Kaihdinpalvelu Ltd | Wholesale & retail | 1998 |
| Oy Suomen LEGO Ab | Wholesale & retail | 1960 |
| Oy Suomen Michelin Ab | Wholesale & retail | 1946 |
| Oy Suomen Onnellisuusmuseo Ab | Arts & recreation | 2025 |
| Oy Suomen Puuyhtymä Ab - Finnish Timber Corporation Ltd - Finnische Holzvereinigung A.G. | Wholesale & retail | 1936 |
| Oy Suomen Rbam Ab | Professional & scientific | 1983 |
| Oy Suomen Tehomax Ab | Professional & scientific | 1995 |
| Oy Suomen Tietotoimisto - Finska Notisbyrån Ab | Information & communication | 1915 |
| Oy Suomen Yritysjuridiikka Ab | Professional & scientific | 1992 |
| Oy Suomi Shipping Services Ltd | Transport & logistics | 1989 |
| Oy Suomi-Neon | Manufacturing | 1961 |
| Oy Super.Natural Nordic Ab | Wholesale & retail | 2012 |
| Oy Superbaum Ab | Financial services | 2026 |
| Oy Suurkeittiö-Insinööritoimisto Rita Pulli - Storköks- Ingenjörsbyrå Ab | Professional & scientific | 1987 |
| Oy Suveka Ab | Wholesale & retail | 2002 |
| Oy Svecasa Ab | Education | 1978 |
| Oy Swepcotec Ab | Construction | 1995 |
| Oy Syd-Finlands Steinhaus Ab | Construction | 2022 |
| Oy Synoptia Ab | Real estate | 1993 |
| Oy Syntema Ab | Wholesale & retail | 1960 |
| Oy Sähkö-Vendelin Ab | Construction | 1997 |
| Oy Söderström & Sandell Ab | Professional & scientific | 1909 |
| Oy T & T Nordcap Ab | Financial services | 2005 |