| Omnishi Shop Limited | Wholesale & retail | 2017 |
| Ompuru Retail Ltd | Wholesale & retail | 2023 |
| Omron Services Limited | Administrative & support | 2003 |
| OMU Smiles Ltd | Health & social care | 2018 |
| Omutumba Investment Ltd | Agriculture & fishing | 2023 |
| OMW Accountancy Ltd | Professional & scientific | 2013 |
| ON a Mission Events Limited | Professional & scientific | 2017 |
| ON a Mission Productions Ltd | Administrative & support | 2024 |
| ON Electrical Ltd | Construction | 2005 |
| ON Line Facilities Management Limited | Construction | 2010 |
| ON Point Appliances Ltd | Wholesale & retail | 2020 |
| ON the Bench Exercise & Massage Limited | Other services | 2010 |
| ON Time Transportation Limited | Transport & logistics | 2024 |
| On-Line Brickwork Limited | Construction | 2009 |
| On-Line Holdings Limited | Transport & logistics | 1992 |
| On-Point Building Contractors Ltd | Construction | 2015 |
| On-Point Property Development Ltd | Real estate | 2018 |
| On-Site Electrical Services Limited | Construction | 1992 |
| ONA Wellbeing Collective Limited | Health & social care | 2025 |
| Onarmor Consulting Ltd | Professional & scientific | 2015 |
| Onboard People Ltd | Manufacturing | 2020 |
| Oncreo Limited | Information & communication | 2024 |
| OND Services Limited | Health & social care | 2015 |
| Ondalab Ltd | Manufacturing | 2025 |
| One Admin Limited | Information & communication | 2024 |
| One Click Vision Ltd | Professional & scientific | 2025 |
| One Click Wonders Ltd | Wholesale & retail | 2023 |
| One Commercial Loans Limited | Financial services | 2022 |
| One Community Hampshire and Dorset Ltd | Health & social care | 2020 |
| One Day Shop Limited | Wholesale & retail | 2022 |
| One Degree West Satellite & Aerial Services Ltd | Wholesale & retail | 2007 |
| One Life Fitness UK Limited | Arts & recreation | 2009 |
| One Stop Minor OP Ltd | Health & social care | 2023 |
| One to One Lettings Limited | Real estate | 2009 |
| One Way Special Projects Ltd | Administrative & support | 2020 |
| Onebowl Rice Noodle Limited | Hospitality | 2025 |
| Onecare (Properties & Investments) Limited | Real estate | 2016 |
| Onecare Group UK Limited | Administrative & support | 2015 |
| Onecare Hampshire Limited | Administrative & support | 2013 |
| Onecare Southampton Limited | Health & social care | 2016 |
| Oneclean 360 (Property Maintenance) Limited | Real estate | 2022 |
| Oneday Solutions Ltd | Transport & logistics | 2026 |
| Onelink Electrical Services Limited | Construction | 2017 |
| Onero Limited | Administrative & support | 2018 |
| Onespace Leisure Ltd | Arts & recreation | 2018 |
| Onespace Management Limited | Real estate | 2016 |
| Oneurl Limited | Information & communication | 2019 |
| Onics Electronics Accessories Ltd | Wholesale & retail | 2024 |
| Onion HR Limited | Administrative & support | 2003 |
| Onit Logistics Group Limited | Transport & logistics | 2019 |
| Online Accountancy and Taxation Limited | Extraterritorial | 2019 |
| Online Car Valuation.Co.Uk Limited | Professional & scientific | 2025 |
| Online Commerce Limited | Wholesale & retail | 2001 |
| Online Payroll Limited | Extraterritorial | 2001 |
| Online Payrolls Limited | Extraterritorial | 2001 |
| Only Commercial Property Limited | Real estate | 2013 |
| Only Rising Properties Ltd | Real estate | 2025 |
| Only Rolls and Sauces Ltd | Other services | 2020 |
| Only YOU Fashion Ltd | Wholesale & retail | 2025 |
| Onlyfries Ltd | Wholesale & retail | 2022 |
| Onpoint Decor Services Limited | Construction | 2025 |
| Onpoint Property Proffessionals Ltd | Construction | 2021 |
| Onsite Images Ltd | Professional & scientific | 2012 |
| Onsite Management Construction Services Limited | Water & waste | 1989 |
| Onsite Weld and Steel Fabrications Limited | Manufacturing | 2021 |
| Onslow Mini Market Limited | Wholesale & retail | 2025 |
| Onslow OFF License Southampton Limited | Wholesale & retail | 2025 |
| Ontime A2Z Ltd | Administrative & support | 2007 |
| Ontrack Projects Ltd | Information & communication | 2025 |
| Onyx Barbers Ltd | Other services | 2026 |
| Onyx Bridal Ltd | Wholesale & retail | 2026 |
| Onyx Ribs Ltd | Manufacturing | 2024 |
| Oomf Ltd | Manufacturing | 2011 |
| Op-Co Marine Ltd | Manufacturing | 2023 |
| Opal Executive Services Ltd | Transport & logistics | 2026 |
| Opcs Design Limited | Information & communication | 2011 |
| Open Cloud Group Ltd | Professional & scientific | 2021 |
| Open Cloud Limited | Professional & scientific | 2023 |
| Open Cloud Media Ltd | Information & communication | 2011 |
| Open Cloud Technology Ltd | Wholesale & retail | 2021 |
| Open Door Carpentry Limited | Construction | 2012 |
| Open Fader Limited | Information & communication | 2015 |
| Open System Solutions Limited | Manufacturing | 2006 |
| Open Technologies Limited | Manufacturing | 1997 |
| Openbills Ltd | Information & communication | 2025 |
| Operaupclose Ltd | Arts & recreation | 2013 |
| Opex Focus Ltd | Professional & scientific | 2023 |
| Opiedax Limited | Other services | 2019 |
| Opient Limited | Information & communication | 2003 |
| Opix Ltd | Professional & scientific | 2019 |
| Oppects Consultancy Services Ltd | Information & communication | 2024 |
| Opsodis Limited | Manufacturing | 2004 |
| Opsonin Enterprise Ltd | Wholesale & retail | 2025 |
| Opsy Sports Ltd | Wholesale & retail | 2026 |
| Optical IT Solutions Limited | Information & communication | 2025 |
| Opticlean Services Limited | Administrative & support | 2025 |
| Optifit Life Ltd | Professional & scientific | 2019 |
| Optiflow Labs Ltd | Information & communication | 2025 |
| Optigroup Holdings Ltd | Financial services | 2025 |
| Optima Carpentry Ltd | Wholesale & retail | 2005 |