| Nvizion Solutions Limited | Information & communication | 2012 |
| NVW Holdco Limited | Financial services | 2020 |
| NWC Holdings Limited | Professional & scientific | 2017 |
| NWD Design Limited | Professional & scientific | 2011 |
| NXT Step Ltd | Wholesale & retail | 2017 |
| Nyakuproperties Limited | Real estate | 2025 |
| Nyingma Life Foundation | Health & social care | 2008 |
| Nyota Tech Ltd | Information & communication | 2025 |
| Nyssa Teleglobal Ltd | Administrative & support | 2014 |
| Nytram Design Limited | Professional & scientific | 2000 |
| Nyvela Property Limited | Administrative & support | 2023 |
| Nzola Properties Limited | Real estate | 2021 |
| O Tuga Ltd | Hospitality | 2020 |
| O'Brien Digital Limited | Administrative & support | 2015 |
| O'Brien Media Retail Ltd | Professional & scientific | 2022 |
| O'Dough's Pizza Co Ltd | Hospitality | 2025 |
| O'Helen Beauty Ltd | Wholesale & retail | 2025 |
| O'Neill Construction Services Ltd | Construction | 2022 |
| Oak & Willow Designs Ltd | Wholesale & retail | 2026 |
| Oak END Way Developments 2 Limited | Other services | 2020 |
| Oak END Way Developments 3 Limited | Real estate | 2020 |
| Oak END Way Developments Limited | Construction | 2013 |
| Oak Furnitureland Group Limited | Wholesale & retail | 2020 |
| Oak Furnitureland Holdings Limited | Financial services | 2020 |
| Oak Hair & Beauty Ltd | Other services | 2024 |
| Oaken Bear Ltd | Information & communication | 2025 |
| Oakes Counselling Limited | Health & social care | 2023 |
| Oakhurst Health Care Ltd | Health & social care | 2022 |
| Oaklands Autos Ltd | Wholesale & retail | 2019 |
| Oakley Park Phase 2 Swindon Management Company Limited | Households | 2007 |
| Oakley Park Swindon Management Company Limited | Households | 2006 |
| Oakridge Building Contractors Ltd | Construction | 2019 |
| Oaksoft Services Ltd | Information & communication | 2011 |
| Oakwood (Swindon) Residents Association Limited | Households | 2001 |
| Oasis Buildings Ltd | Real estate | 2026 |
| Oasis Garden Care Ltd | Other services | 2024 |
| OB Chef Services Ltd | Hospitality | 2025 |
| OBA Lola Ventures Ltd | Professional & scientific | 2021 |
| Oban 24 Ltd | Construction | 2016 |
| Obatiwa Limited | Health & social care | 2020 |
| Objects of Class Limited | Information & communication | 1995 |
| OBL Property Management Ltd | Real estate | 2020 |
| Obsidian Auto Exchange Ltd | Wholesale & retail | 2025 |
| Obszynscy Property Ltd | Real estate | 2024 |
| Obszynski Limited | Transport & logistics | 2012 |
| Obszynski Property Solutions Ltd | Construction | 2026 |
| Obvious Technology Limited | Information & communication | 2011 |
| OC Estates and Developments Limited | Real estate | 2021 |
| OCC Plant Ltd | Administrative & support | 2016 |
| Ocean Breeze Freeholders Limited | Households | 2017 |
| Ocean IT Services Ltd | Information & communication | 2025 |
| Ocean View Cottages Limited | Real estate | 2020 |
| Ocean View Healthcare Ltd | Health & social care | 2025 |
| Ocella Holdings Limited | Information & communication | 1993 |
| Ocella Properties Ltd | Construction | 2020 |
| Ocella Software Systems Limited | Information & communication | 1993 |
| Ockwell Consulting Limited | Information & communication | 2006 |
| OCT7 Ltd | Manufacturing | 2025 |
| Octal Data Solutions Limited | Information & communication | 2021 |
| Octave Intelligence Finance Ltd | Professional & scientific | 2026 |
| Octillion World Ltd | Information & communication | 2024 |
| Octohealth Ltd | Health & social care | 2022 |
| Octus Global Limited | Professional & scientific | 2013 |
| Oculus Wealth Management (Wiltshire) Limited | Financial services | 2017 |
| Odikrok Care Services Limited | Health & social care | 2019 |
| Odin Darts Ltd | Manufacturing | 2025 |
| Odin SECURITY1 Ltd. | Administrative & support | 2021 |
| OE Delight Limited | Hospitality | 2024 |
| Offer Roofing Services Limited | Construction | 2013 |
| Offgrid Services Limited | Professional & scientific | 2015 |
| Offices for YOU Swindon Ltd | Real estate | 2019 |
| Offlayer Core Ltd | Information & communication | 2026 |
| Ofie Direct Ltd | Transport & logistics | 2018 |
| Ofie Limited | Hospitality | 2021 |
| OG Groundworks Limited | Construction | 2025 |
| OG Life Studios Ltd | Information & communication | 2024 |
| Ogle-Skan Consulting Ltd | Professional & scientific | 2024 |
| OH Hello Bricks Ltd | Wholesale & retail | 2020 |
| Ohene Multimedia Ltd | Information & communication | 2023 |
| Ohja Limited | Information & communication | 2017 |
| Oikos Digital Ltd | Information & communication | 2013 |
| Oink Gallery Ltd | Wholesale & retail | 2016 |
| OJ Auto Services Ltd | Wholesale & retail | 2025 |
| OJS Advisory Limited | Wholesale & retail | 2025 |
| OJW Consulting Ltd | Professional & scientific | 2025 |
| OK Commercial Limited | Financial services | 2025 |
| OK Residential Limited | Real estate | 2026 |
| Okebourne Transport Solutions Ltd | Professional & scientific | 2018 |
| Okells Law Limited | Professional & scientific | 2010 |
| Okells With Francislaw LLP | | 2006 |
| Okodie Holdings Limited | Real estate | 2024 |
| Okoli Property Group Ltd | Real estate | 2024 |
| Okudaaibube Ltd | Health & social care | 2021 |
| Okus Construction Limited | Construction | 2008 |
| Okus Convenience Store Ltd | Wholesale & retail | 2012 |
| Ola's Cakehouse Ltd | Administrative & support | 2025 |
| Olac Limited | Real estate | 2012 |
| Olas Healthcare Limited | Construction | 2016 |
| Olauda Properties Limited | Real estate | 2017 |
| Olauda Serviced Accommodation Ltd | Hospitality | 2019 |