| Optive Media Group Limited | Professional & scientific | 2025 |
| Optomistry Limited | Wholesale & retail | 2015 |
| Optp Express Ltd | Hospitality | 2023 |
| Optp HUT Ltd | Hospitality | 2024 |
| Opulent UK Ltd | Wholesale & retail | 2019 |
| Opulent Voyage Ltd | Other services | 2019 |
| Opulenzo Ltd | Wholesale & retail | 2026 |
| Opus Entertainment Group Ltd | Hospitality | 2021 |
| Opus Merp UK Limited | Financial services | 2022 |
| Oracle Security Services Ltd | Administrative & support | 2015 |
| Oracul Ltd | Professional & scientific | 2006 |
| Orakzai Traders Limited | Transport & logistics | 2020 |
| Oral Pathways Ltd | Information & communication | 2025 |
| Orange Core Ltd | Manufacturing | 2012 |
| Orange Orchid Healthcare Limited | Administrative & support | 2017 |
| ORB Consult Ltd | Health & social care | 2024 |
| ORB Energy Ltd | Information & communication | 2023 |
| Orbit Ilford Ltd | Real estate | 2021 |
| Orbit London Ltd | Real estate | 2017 |
| Orbit Properties London Ltd | Real estate | 2016 |
| Orbital Capitals Limited | Real estate | 2020 |
| Orbitx Logistics Limited | Transport & logistics | 2025 |
| Orbule Ltd | Information & communication | 2026 |
| Orca PM Consulting Limited | Administrative & support | 2016 |
| Orchard Care Agency Limited | Health & social care | 2013 |
| Orchard Court Developments London Ltd | Real estate | 2016 |
| Orchard LEA Homes Ltd | Construction | 2026 |
| Orchard Medical Limited | Health & social care | 2019 |
| Orchid Realty Ltd | Real estate | 2024 |
| Orchid Solicitors Ltd | Professional & scientific | 2017 |
| Ordevo Limited | Information & communication | 2020 |
| Ordina Home Ltd | Wholesale & retail | 2026 |
| Orellion Limited | Wholesale & retail | 2021 |
| Orga Properties Ltd | Households | 2025 |
| Organic POD Limited | Wholesale & retail | 2017 |
| Organic Products Limited | Hospitality | 2025 |
| Organisation of Certified Journalists Limited | Education | 2025 |
| Organisation of Certified Risk Managers Limited | Information & communication | 2023 |
| Orgnic Lifestyle Limited | Manufacturing | 2019 |
| Orhan Creations Ltd | Wholesale & retail | 2024 |
| Oriel Financial Limited | Information & communication | 2014 |
| Oriens Healthcare Limited | Health & social care | 2020 |
| Orient Asia Ltd | Wholesale & retail | 2021 |
| Orient Auto Parts Ltd | Wholesale & retail | 2018 |
| Orient Properties Services Limited | Construction | 2019 |
| Orient Property Services Ltd | Real estate | 2011 |
| Orient Traders Ltd | Wholesale & retail | 2017 |
| Oriental Chef (Edmonton) Limited | Wholesale & retail | 2020 |
| Oriental Chef (Wanstead) Limited | Hospitality | 2015 |
| Oriental Food (West Green Road) Limited | Hospitality | 2024 |
| Oriental T J Limited | Wholesale & retail | 2022 |
| Oriental Xpress Ltd | Hospitality | 2023 |
| Origen Groups Limited | Real estate | 2025 |
| Origin Beauté Ltd | Wholesale & retail | 2026 |
| Original Foods (London) Ltd | Hospitality | 2021 |
| Original King's Caliphs Ltd | Wholesale & retail | 2019 |
| Original Lahore Express Limited | Hospitality | 2025 |
| Orignal Lahore Express Limited | Hospitality | 2025 |
| Orion Consulting (UK) Ltd | Professional & scientific | 2015 |
| Orion Consulting Solutions Limited | Information & communication | 2017 |
| Orion Couriers Ltd | Transport & logistics | 2025 |
| Orionis Digital Services Limited | Information & communication | 2019 |
| Oriontech Ltd | Information & communication | 2013 |
| Orix Services Ltd | Administrative & support | 2017 |
| Orizon Ltd | Wholesale & retail | 2026 |
| Ormi Transfer Limited | Transport & logistics | 2025 |
| Ormiston House Consulting and Writing Services Ltd | Information & communication | 2016 |
| Ormix Limited | Financial services | 2025 |
| Ornate ELM Kitchen and Bedrooms Ltd | Construction | 2025 |
| Ornate Tiles Ltd | Manufacturing | 2020 |
| Orpi Events Ltd | Administrative & support | 2025 |
| Orrelin Limited | Wholesale & retail | 2026 |
| Orren Limited | Wholesale & retail | 2025 |
| Orspine Limited | Professional & scientific | 2013 |
| Orsum Products Ltd | Wholesale & retail | 2026 |
| Ortek Global Limited | Energy | 2017 |
| Orthochandra Limited | Health & social care | 2020 |
| Oruc Cleaning Services Ltd | Administrative & support | 2019 |
| Orwin Entertainment Group Ltd | Hospitality | 2019 |
| OS UK Eggs Limited | Wholesale & retail | 2014 |
| Os,S Services Ltd | Administrative & support | 2024 |
| Os-Intelligence Commercials Ltd | Other services | 2024 |
| Osbond and Tutt Limited | Other services | 2000 |
| Osborne Woodcare Ltd | Construction | 2014 |
| Osei Management & Consulting Ltd | Professional & scientific | 2021 |
| Oshqual Ltd | Information & communication | 2024 |
| Osil London Ltd | Real estate | 2016 |
| Osker Enterprises Limited | Other services | 2022 |
| Oslid Ltd | Administrative & support | 2024 |
| Osman Timber Supplies Limited | Wholesale & retail | 2026 |
| Osmodal Emea Ltd | Transport & logistics | 2025 |
| Osprey Facilities Ltd | Wholesale & retail | 2024 |
| Ostedal Limited | Construction | 2022 |
| Ostial Investments Limited | Real estate | 2024 |
| Ostler Home Counties Limited | Other services | 2017 |
| Ostro Residencies Limited | Real estate | 2024 |
| Osus Holdings Limited | Wholesale & retail | 2022 |
| Osus Store Limited | Wholesale & retail | 2022 |
| Osymar UK Limited | Administrative & support | 2020 |
| Other Palette CIC | Arts & recreation | 2023 |