| Ornsby Building & Construction Limited | Construction | 2021 |
| Ornsby Electrical Limited | Manufacturing | 2015 |
| Ornsby Group Limited | Financial services | 2021 |
| Orras Holdings Limited | Financial services | 2024 |
| Orseis Ltd | Information & communication | 2026 |
| Orthodontic and Property Services Limited | Real estate | 2020 |
| Ortur Limited | Administrative & support | 2024 |
| Orvex Ltd | Wholesale & retail | 2025 |
| OS Constructions Limited | Construction | 1998 |
| Osaat Limited | Energy | 2023 |
| Osborne House Freehold Limited | Households | 2014 |
| Osborne House Management Company (Southampton) Limited | Households | 1991 |
| Oscar & Co Meadows Ltd | Agriculture & fishing | 2026 |
| Oscar James Mechanical Ltd | Construction | 2022 |
| Oscar Mavunga Limited | Health & social care | 2025 |
| Oscorp Technology Limited | Information & communication | 2024 |
| Oska Hair Ltd | Other services | 2004 |
| OSM Distribution Ltd | Transport & logistics | 2026 |
| Osman Homes Limited | Real estate | 2017 |
| Osman Plumbing & Heating Ltd | Construction | 2024 |
| Osmotech U.K. Group Limited | Financial services | 2023 |
| Osmowash Extreme Cleaning Services Limited | Administrative & support | 2013 |
| Osmowash Group Limited | Administrative & support | 2021 |
| Osprey Furnishings Limited | Wholesale & retail | 1998 |
| Osprey Roofing Ltd | Construction | 2024 |
| Osprey Technical Consulting Ltd | Manufacturing | 2012 |
| Osrc (Norway) Limited | Other services | 2012 |
| Osrc (South Africa) Limited | Other services | 2012 |
| Ossory Astwood Limited | Real estate | 1998 |
| Ossory Property Investments Limited | Real estate | 1989 |
| OST Engineering Limited | Administrative & support | 2011 |
| Osteonorm Ltd | Hospitality | 2016 |
| Ostlers Yard Management Limited | Households | 2021 |
| Ostman Homes Ltd | Construction | 2023 |
| OTB Repair Ltd | Other services | 2022 |
| Othens Commercials Ltd | Manufacturing | 2022 |
| Othens Transport Limited | Transport & logistics | 2004 |
| Other Skies Ltd | Professional & scientific | 2021 |
| Otis Fraser Ltd | Professional & scientific | 2019 |
| OTJ Engineering Ltd | Administrative & support | 2024 |
| OTR Associates Ltd | Professional & scientific | 2019 |
| Ottelo Electrical Ltd | Construction | 2026 |
| Ottilie Ltd | Other services | 2026 |
| OUD Studio Limited | Wholesale & retail | 2024 |
| OUR Gate Limited | Real estate | 2025 |
| OUR Higher Being Ltd | Other services | 2024 |
| OUR IT Space Ltd | Information & communication | 2025 |
| OUR Tiny Marketing Co Limited | Information & communication | 2013 |
| OUR Version Media C.I.C. | Information & communication | 2019 |
| Ourheat Limited | Construction | 2014 |
| Ouroboros MMA Ltd | Arts & recreation | 2023 |
| Oursophia Limited | Wholesale & retail | 2019 |
| Ousia Business Solutions Ltd | Administrative & support | 2026 |
| OUT and OUT Solutions Limited | Professional & scientific | 1997 |
| Outcome (Bournemouth) Limited | Construction | 2024 |
| Outcomehome C.I.C. | Health & social care | 2012 |
| Outdoor BBQ Kitchens Limited | Wholesale & retail | 2026 |
| Outermost Apparel Ltd | Wholesale & retail | 2020 |
| Outlandish Digital Ltd | Wholesale & retail | 2021 |
| Outlandish Logistics Limited | Transport & logistics | 2025 |
| Outlaws BW Ltd | Other services | 2022 |
| Outlove Ltd | Professional & scientific | 2024 |
| Outmor Ltd | Wholesale & retail | 2025 |
| Outpace Ltd | Professional & scientific | 2025 |
| Outrider Operations Ltd | Information & communication | 2026 |
| Outside Investments Limited | Real estate | 2025 |
| Outwood Cars Limited | Administrative & support | 2004 |
| Outwood Sports & Classic Ltd | Administrative & support | 2020 |
| Ouzal Limited | Transport & logistics | 2023 |
| Oven Doctor Southampton Limited | Administrative & support | 2025 |
| Over-Ear Limited | Extraterritorial | 2012 |
| Overheat Digital Ltd | Information & communication | 2021 |
| Overknight Ltd | Real estate | 2019 |
| Overrun Media Ltd | Information & communication | 2017 |
| Overson Limited | Arts & recreation | 1996 |
| Overton Flats Residents Limited | Households | 1990 |
| Overwell Services Limited | Professional & scientific | 2003 |
| OVH Property Limited | Real estate | 2026 |
| OVH Trading Limited | Hospitality | 2025 |
| Ovidiu Trans Express Ltd | Transport & logistics | 2025 |
| Oviisoft Limited | Information & communication | 2015 |
| Ovington Tichborne (Owners) Limited | Households | 2017 |
| OVM Eleven Limited | Real estate | 2015 |
| OVP Consult Ltd | Administrative & support | 2024 |
| OVY Transport Ltd | Transport & logistics | 2025 |
| OW Detailing Ltd | Administrative & support | 2026 |
| Owen Electrical Solutions Limited | Construction | 2016 |
| Owen Nurseries Limited | Education | 2005 |
| Owens Marine Services Ltd | Manufacturing | 2026 |
| Owls Road Management Company Limited | Households | 2003 |
| Owls Surveillance Limited | Manufacturing | 2014 |
| Owlsight Security Ltd | Administrative & support | 2025 |
| OWS UK Holdings Ltd | Financial services | 2024 |
| Owslebury Investments Limited | Professional & scientific | 2021 |
| Owtons Holding Limited | Financial services | 2018 |
| OX Brasserie Ltd | Hospitality | 2013 |
| Ox-Seven Limited | Administrative & support | 2018 |
| Oxbrook Properties Ltd | Real estate | 2024 |
| Oxenburys Yard Limited | Construction | 2024 |
| Oxentra Limited | Wholesale & retail | 2025 |