| Old Cooperage Distribution Limited | Wholesale & retail | 2025 |
| Old Court Management Limited | Households | 2016 |
| Old Cryptians RFC Limited | Arts & recreation | 2019 |
| Old Orchard Close (Christian Malford) Management Company Limited | Households | 2024 |
| Old Town Accountancy Ltd | Professional & scientific | 2012 |
| Old Town Aesthetics Ltd | Other services | 2025 |
| Old Town Business Association Limited | Administrative & support | 2020 |
| Old Town Chiropractic Clinic Ltd | Health & social care | 2024 |
| Old Town Hardware Limited | Wholesale & retail | 2006 |
| Old Town Independent Financial Advice Limited | Financial services | 2022 |
| Old Town Local Management Company Limited | Households | 2013 |
| Old Town Property and Residential Ltd | Real estate | 2021 |
| Old Town Property Developments Limited | Real estate | 2007 |
| Old Town Station Industrial Estate Management Company Limited | Real estate | 1982 |
| Olid Holdings Limited | Professional & scientific | 2023 |
| Olim Consulting Ltd | Information & communication | 2017 |
| Olive & Co Properties Ltd | Administrative & support | 2025 |
| Olive & Stone Properties Ltd | Real estate | 2025 |
| Olive Wood Group Limited | Information & communication | 2022 |
| Olive Wood IT Limited | Information & communication | 2022 |
| Oliveira Constructions Ltd | Construction | 2022 |
| Oliveira's Butcher & Grill Ltd | Hospitality | 2026 |
| Oliver Christian Properties Ltd | Real estate | 2014 |
| Oliver Curran Construction Limited | Construction | 2002 |
| Oliver Hope Kitchens Ltd | Construction | 2014 |
| Oliver Mcfoy Services Limited | Information & communication | 2022 |
| Oliveris Tech Accelerator Ltd | Professional & scientific | 2022 |
| Olivia KYI Maung Ltd | Health & social care | 2026 |
| Ollie O'Neill Wall and Floor Tiling Limited | Construction | 2011 |
| Olliegroup Ltd | Construction | 2019 |
| Ollivion Ltd | Manufacturing | 2022 |
| Olson Consulting Limited | Professional & scientific | 2013 |
| Olucrystal Healthcare Services Limited | Health & social care | 2022 |
| Olympias Services Ltd | Administrative & support | 2024 |
| Olympus Business Management Ltd | Professional & scientific | 2015 |
| Olympus Electrical Wholesale Limited | Wholesale & retail | 2009 |
| OM Asset Management Limited | Real estate | 2022 |
| OM Solutions Limited | Information & communication | 2012 |
| OM Sree First Property Ltd | Real estate | 2023 |
| Omega Electrical Services Swindon Ltd | Energy | 2024 |
| Omega Financial Brokers Ltd | Financial services | 2018 |
| Omega Invest Ltd | Real estate | 2021 |
| Omega Media Consultancy Ltd | Information & communication | 2012 |
| Omega Property Experts Ltd | Real estate | 2021 |
| OML Support Ltd | Health & social care | 2025 |
| Omni Channel Fulfilment Limited | Transport & logistics | 2017 |
| Omni Roots Hub CIC | Education | 2026 |
| Omnia Vinyl Ltd | Other services | 2017 |
| Omniadigital and Data Ltd | Information & communication | 2020 |
| Omnis Investments Limited | Financial services | 2008 |
| Omren Properties Limited | Real estate | 2026 |
| ON Call Practice Management Limited | Administrative & support | 2013 |
| ON Schedule Events Limited | Administrative & support | 2015 |
| ON Way Home Limited | Hospitality | 2024 |
| On-Site Landscaping Limited | Construction | 2007 |
| Onbelay Climbing (Hemel) Ltd | Arts & recreation | 2026 |
| ONC Scaffolding Limited | Construction | 1993 |
| One and Done Ltd | Construction | 2025 |
| One Call Cleaning Ltd | Administrative & support | 2020 |
| One Continuum Limited | Professional & scientific | 2006 |
| One Forty Three Properties Limited | Real estate | 2024 |
| One O One Convenience Store Swindon Ltd | Wholesale & retail | 2024 |
| One Planet Limited | Professional & scientific | 2015 |
| One Step Joinery Limited | Manufacturing | 1998 |
| One Stop Driving Academy Ltd | Education | 2025 |
| One Stop Staffing Limited | Administrative & support | 2018 |
| One Swindon Limited | Administrative & support | 2016 |
| ONE2ONEDECORATORS Limited | Construction | 2015 |
| Onefairview Limited | Construction | 2015 |
| Onefortynine Games Ltd | Information & communication | 2026 |
| Onehealth 4 ALL Limited | Health & social care | 2024 |
| Oneide's Cleaning Services Ltd | Administrative & support | 2025 |
| Onel Real Estate UK Limited | Real estate | 2011 |
| Onepointenterprise Ltd | Information & communication | 2023 |
| Oneroof Stays Ltd | Real estate | 2025 |
| Oneway Living Limited | Real estate | 2018 |
| Onksoft Ltd | Information & communication | 2025 |
| Only Human Limited | Administrative & support | 2025 |
| Only Scale Limited | Other services | 2024 |
| Onna Travel Ltd | Transport & logistics | 2019 |
| Onrao Ltd | Transport & logistics | 2022 |
| Onsite Truck Body Repairs Ltd | Wholesale & retail | 2014 |
| Ontime Staffing Solutions Ltd | Administrative & support | 2023 |
| Onyibee African Food Ltd | Hospitality | 2025 |
| Onyx Builder Ltd | Construction | 2022 |
| Onyx ELK Properties Ltd | Construction | 2026 |
| Onyx Plumber Ltd | Construction | 2025 |
| Opah Consulting Limited | Professional & scientific | 2026 |
| Opel Construction Ltd | Construction | 2019 |
| Open Arms Healthcare Ltd | Health & social care | 2020 |
| Open Arms Healthcare Solutions Limited | Health & social care | 2022 |
| Open Dawes Training Ltd | Professional & scientific | 2023 |
| Open Systems Informer Limited | Information & communication | 1986 |
| Open the Book | Extraterritorial | 2011 |
| Open View Tech Ltd | Manufacturing | 2024 |
| Open Water Swimming Lake 59 Ltd | Arts & recreation | 2020 |
| OPENINSIGHT101 Limited | Information & communication | 2016 |
| Openroad Projects Ltd | Professional & scientific | 2018 |
| Openwork Access Limited | Administrative & support | 2004 |
| Openwork Holdings Limited | Professional & scientific | 1999 |