| Offbeet Limited | Hospitality | 2017 |
| Office Complete Limited | Wholesale & retail | 2025 |
| Office Complete Recycling Limited | Wholesale & retail | 2025 |
| Office of Stephen Parker Ltd | Administrative & support | 2021 |
| OFFICE4YOU Limited | Administrative & support | 2016 |
| Officebnb Limited | Administrative & support | 2012 |
| Offset Clearing House Limited | Other services | 2024 |
| Offset Customer Services Limited | Information & communication | 2024 |
| Offset Holdings Limited | Financial services | 2024 |
| OFS Consulting Ltd | Information & communication | 2026 |
| OG Marine Limited | Manufacturing | 2023 |
| Ogen Management Limited | Professional & scientific | 2024 |
| Ogen Properties Limited | Financial services | 2024 |
| OGF Consulting Limited | Professional & scientific | 2015 |
| OGS Services Ltd | Financial services | 2025 |
| Ohco Coffee Ltd | Manufacturing | 2021 |
| Ohdeer Ltd | Hospitality | 2024 |
| OHM Projects Ltd | Professional & scientific | 2025 |
| Oisín Creative Ltd | Wholesale & retail | 2018 |
| OJD Pictures Ltd | Arts & recreation | 2020 |
| OLA DOR 2003 Ltd | Administrative & support | 2017 |
| Old Farm Contracting Limited | Extraterritorial | 2008 |
| Old Farm Court (Nursling) Management Company Limited | Households | 2019 |
| Old Fashioned Sweetie Shop Ltd | Wholesale & retail | 2024 |
| Old Town 26 Ltd | Hospitality | 2026 |
| Oldchamps Barbers Ltd | Other services | 2022 |
| Olde Willow Ltd | Information & communication | 2015 |
| Olivaricore Ltd | Information & communication | 2025 |
| Oliver Chandler Limited | Real estate | 2017 |
| Oliver Mews (Winchester) Management Company Limited | Extraterritorial | 2015 |
| Olivi Traders Limited | Hospitality | 2015 |
| Ollibees Media Limited | Professional & scientific | 2021 |
| OM Hospitality Ltd | Hospitality | 2026 |
| Omega Design Consultants LLP | | 2013 |
| Omni Digital Technologies Ltd | Information & communication | 2017 |
| Omnimed Limited | Administrative & support | 2008 |
| Omnimus Limited | Wholesale & retail | 2017 |
| Omodelta Ltd | Hospitality | 2025 |
| ON My Plate Limited | Manufacturing | 2024 |
| ON Sale When Ltd | Information & communication | 2025 |
| ON the Menu Ltd | Manufacturing | 2010 |
| One Degree North (Eastleigh) Management Company Limited | Households | 2005 |
| One Drop Media Limited | Information & communication | 2020 |
| One Eyed Cat Games Ltd | Information & communication | 2025 |
| One Life Learning Limited | Education | 2010 |
| One More Pouch Ltd | Wholesale & retail | 2026 |
| One Ninety UK Limited | Wholesale & retail | 2007 |
| One of the People Ltd | Wholesale & retail | 2024 |
| ONE90 Limited | Construction | 2020 |
| Onevision Systems Ltd | Wholesale & retail | 2025 |
| Ongama Limited | Health & social care | 2022 |
| Onslow Auctions Limited | Other services | 1984 |
| Ontap Bathrooms Ltd | Construction | 2004 |
| Ooooby Hampshire Ltd | Wholesale & retail | 2020 |
| Opalrock Winchester Limited | Real estate | 2024 |
| Open House Deli Limited | Hospitality | 2018 |
| Open Studio Art School Ltd | Education | 2025 |
| Openstream Limited | Information & communication | 1992 |
| Opex Resources Ltd | Information & communication | 2010 |
| Ophthalmedica Limited | Manufacturing | 1983 |
| OPS Innovations Ltd | Information & communication | 2014 |
| Optimaux Ltd | Energy | 2019 |
| Optimisare Ltd | Information & communication | 2024 |
| Optimising Potential Ltd | Professional & scientific | 2022 |
| Optomel Limited | Manufacturing | 2018 |
| Optrix Ltd | Professional & scientific | 2025 |
| OPW Properties Limited | Real estate | 2016 |
| Orangetreeprojects Ltd | Financial services | 2024 |
| Orban Motor Co Ltd | Wholesale & retail | 2022 |
| Orchard Court Crondall Residents Association Limited | Households | 1983 |
| Orchard Place (Winchester) Limited | Households | 2016 |
| Orchard Wealth Limited | Financial services | 2014 |
| Orchidweb Limited | Information & communication | 1998 |
| Ordinal FD Ltd | Construction | 2017 |
| ORE & Metal International Limited | Mining & quarrying | 2022 |
| Organic Soft Cheese Ltd | Extraterritorial | 2006 |
| Origo Trans Ltd | Transport & logistics | 2019 |
| Orlov Construction Limited | Construction | 2017 |
| Ormantine International Limited | Manufacturing | 1977 |
| Ormonde Gate Investments Limited | Financial services | 2015 |
| Orthodontics by EVA Ltd | Health & social care | 2016 |
| Orthodontics by EVA Petersfield & Alton Limited | Health & social care | 2016 |
| Orthodontics by EVA Winchester Ltd | Health & social care | 2016 |
| Orthopticus Limited | Health & social care | 2026 |
| Ortibly Property Management Ltd | Other services | 2012 |
| Orwell Independent Ltd | Professional & scientific | 2022 |
| Osborne Ascent Ltd | Health & social care | 2026 |
| OSC Studio Ltd | Professional & scientific | 2017 |
| Osinto Software Ltd | Information & communication | 2019 |
| Osmosis Studios Limited | Real estate | 2025 |
| Osprey Flight Solutions Ltd | Information & communication | 2020 |
| Otae Investments Limited | Real estate | 2020 |
| Otae Properties Limited | Real estate | 2022 |
| Otterbourne Farm Limited | Construction | 2008 |
| Ottino Bookkeeping Services Limited | Professional & scientific | 2008 |
| Ottino's Limited | Hospitality | 2012 |
| OUR Evolution Ltd | Other services | 2026 |
| OUR Only Planet Ltd | Wholesale & retail | 2020 |
| OUR Potential Limited | Administrative & support | 2007 |
| Outside HQ Ltd | Information & communication | 2022 |