| Old Hanging Wood CIC | Agriculture & fishing | 2020 |
| Old School Mews (Ashford) Management Company Limited | Households | 2016 |
| Old Vicarage Gardens (WYE) Management Limited | Households | 1998 |
| Old Willow Properties Limited | Real estate | 2016 |
| Oldfield Place Management Company Limited | Households | 2026 |
| Oldfields Newsagents Limited | Wholesale & retail | 2011 |
| Olite Academy Ltd | Education | 2025 |
| Olive Tree Properties Ltd | Real estate | 2024 |
| Olive Tree Veterinary Surgery Ltd | Professional & scientific | 2024 |
| Oliver Barat Wealth Management Limited | Financial services | 2018 |
| Oliver Gray Contract Services Limited | Construction | 2024 |
| Oliver Sutton Limited | Professional & scientific | 2017 |
| Oliver Turner & Co Construction Ltd | Construction | 2025 |
| Oliver Turrell's Limited | Wholesale & retail | 2024 |
| Olivia Cottages Residential Management Company Limited | Households | 2017 |
| Olly Knight Photography Limited | Arts & recreation | 2013 |
| Olojo Limited | Health & social care | 2021 |
| Olson Property Investments Limited | Real estate | 2022 |
| Olson Timber Buildings Limited | Construction | 2017 |
| Olympus Support Services Ltd | Professional & scientific | 2021 |
| OM Computers Ltd | Information & communication | 2008 |
| OM Namo Namah Ltd | Hospitality | 2018 |
| Omatofe Ltd | Health & social care | 2017 |
| OMC Data Ltd | Information & communication | 2025 |
| Omega Conferences & Events Limited | Administrative & support | 2022 |
| Omega Traffic Services Ltd | Construction | 2019 |
| OMI Engineering Ltd | Professional & scientific | 2024 |
| Omicron Medical Consulting Limited | Professional & scientific | 2020 |
| Omni Graphics Limited | Professional & scientific | 2022 |
| Omni Holdings Group Limited | Financial services | 2025 |
| Omni Properties Limited | Real estate | 2025 |
| Omnikare Limited | Professional & scientific | 2015 |
| Omvara Holdings & Ventures Limited | Wholesale & retail | 2025 |
| ON SET Greens Ltd | Administrative & support | 2023 |
| Onboard Transport Limited | Transport & logistics | 2022 |
| Oncall FIX Ltd | Wholesale & retail | 2026 |
| Once Upon a Wrinkle Ltd | Other services | 2024 |
| One Door Ltd | Wholesale & retail | 2024 |
| One Lombard International Limited | Financial services | 2021 |
| One Lords Court Ltd | Real estate | 2016 |
| One Lords Court Residential Limited | Real estate | 2012 |
| One of a Kind Ltd | Wholesale & retail | 2025 |
| One Strategist Ltd | Professional & scientific | 2023 |
| One Summitt Limited | Financial services | 2021 |
| One Thirty (Kent) Limited | Construction | 2024 |
| Oneaim.Com Limited | Information & communication | 1999 |
| Onedigital Consulting Ltd | Information & communication | 2025 |
| Onefold Digital Limited | Information & communication | 2015 |
| Onehoursitefix Limited | Information & communication | 2014 |
| Oneoff Records Ltd | Arts & recreation | 2025 |
| Onestop Fish & Chips Ltd | Hospitality | 2016 |
| Onetwo Group Consultancy Ltd | Extraterritorial | 2022 |
| Onetwo Medical Ltd | Professional & scientific | 2022 |
| Onground Properties Ltd | Real estate | 2025 |
| Onground Transfers Ltd | Transport & logistics | 2024 |
| Onimi Ltd | Health & social care | 2023 |
| Onionai Ltd | Information & communication | 2023 |
| Oniq Properties Ltd | Real estate | 2025 |
| Onit Lofts Ltd | Construction | 2020 |
| Online Experts Limited | Extraterritorial | 2009 |
| Online Quality Shop Ltd | Manufacturing | 2024 |
| Onqor Ltd | Information & communication | 2017 |
| Onset Projects Ltd | Manufacturing | 2023 |
| Onthesquare Building Compliance Ltd | Construction | 2019 |
| Onus Projects Ltd | Construction | 2024 |
| OO Enterprises Limited | Financial services | 2023 |
| Oola Limited | Administrative & support | 2022 |
| OP Building Ltd | Construction | 2021 |
| Opak Limited | Wholesale & retail | 2012 |
| Opal Art Guild Ltd | Wholesale & retail | 2025 |
| Opala Ltd | Wholesale & retail | 2013 |
| OPB Ltd | Professional & scientific | 2020 |
| Open Air Limited | Hospitality | 2017 |
| Open Future Consulting Limited | Information & communication | 2008 |
| Open Sourcery Ltd | Information & communication | 2017 |
| Operational Accounting Systems Limited | Information & communication | 2004 |
| Operator Training Services Limited | Public administration | 2006 |
| OPM Engineering Limited | Construction | 2024 |
| Opticx Ltd | Wholesale & retail | 2024 |
| Optima Infra Ltd | Real estate | 2025 |
| Optima Property Management Limited | Real estate | 2013 |
| Optimising Health of the Over 50’S Limited | Professional & scientific | 2025 |
| Optimum Flooring Limited | Construction | 2019 |
| Optimum Inceptum Ltd | Hospitality | 2022 |
| Options Exhibitions and Displays Limited | Other services | 2000 |
| Optivian Group Ltd | Professional & scientific | 2022 |
| Opulence AI Agency Ltd | Information & communication | 2025 |
| Opulence Housing Limited | Real estate | 2023 |
| Opulence Online Retail Ltd | Wholesale & retail | 2024 |
| Opulent Mind Limited | Professional & scientific | 2023 |
| Opulent Rides Ltd | Extraterritorial | 2024 |
| Opus Care Developments Limited | Construction | 2020 |
| Opus Care Holdings Limited | Financial services | 2020 |
| Opus Care Investments Limited | Real estate | 2020 |
| Opus Care Limited | Health & social care | 1998 |
| Opus Cleaning Services Limited | Administrative & support | 2022 |
| Opushaven Group Ltd | Professional & scientific | 2024 |
| Orah Creative Group Ltd | Education | 2025 |
| Orange Construction (UK) Ltd | Construction | 2013 |
| Orange Lifecare Limited | Health & social care | 2024 |