| Oskit Developments Limited | Construction | 2024 |
| Oslaw Creations Limited | Manufacturing | 2023 |
| OSM Ventilation Limited | Manufacturing | 2025 |
| Osma Consulting Ltd | Professional & scientific | 2022 |
| Osmani Cars Ltd | Administrative & support | 2024 |
| Osmanov Properties Ltd | Real estate | 2020 |
| Osmanov Services Ltd | Health & social care | 2019 |
| Osmii Limited | Administrative & support | 2011 |
| OSO Holdings Ltd | Information & communication | 2019 |
| OSO Lindo Properties Ltd | Real estate | 2022 |
| OSP Technical (UK) Ltd | Professional & scientific | 2011 |
| OSP Technology Ltd | Information & communication | 2019 |
| Osprey Offices LLP | | 2023 |
| Osprey Professional Services Ltd | Information & communication | 2016 |
| Osprey Tech Limited | Information & communication | 2019 |
| Osprey Testing Limited | Information & communication | 1993 |
| Osram Limited | Manufacturing | 2015 |
| OSS Technologies Limited | Financial services | 2024 |
| Osseo Parts Limited | Manufacturing | 2019 |
| Ossett Car Sales Ltd | Wholesale & retail | 2018 |
| Ossett Gyros Deliveries Ltd | Hospitality | 2025 |
| Osspharm Limited | Wholesale & retail | 2024 |
| Ostacom Ltd | Transport & logistics | 2018 |
| Osuyi Limited | Health & social care | 2020 |
| Osvaldas Gesventas Ltd | Financial services | 2025 |
| Oswald Chambers Publications Association Limited(The) | Information & communication | 1942 |
| Oswald Holdings Limited | Real estate | 2020 |
| Oswestry Radiologists LLP | | 2022 |
| OT Dog Limited | Real estate | 2025 |
| Ot&T Limited | Administrative & support | 2025 |
| Ot&T Logistics Limited | Transport & logistics | 2026 |
| Otaku Property Group Limited | Real estate | 2016 |
| OTE Sports Limited | Wholesale & retail | 2012 |
| Oteva Limited | Professional & scientific | 2015 |
| Otfm Commercial Solar Limited | Construction | 2025 |
| Otgl Limited | Information & communication | 2016 |
| Other Things Ltd | Professional & scientific | 2019 |
| Otherlook Limited | Real estate | 1986 |
| Otherworldly Interiors Ltd | Wholesale & retail | 2025 |
| Othinn Limited | Other services | 2024 |
| Otimo Services Ltd | Administrative & support | 2025 |
| Otis & Barr Ltd | Other services | 2024 |
| Otium Investment Company Ltd | Real estate | 2019 |
| Otiumedge Ltd | Information & communication | 2025 |
| Otley Garden Machinery Ltd | Wholesale & retail | 2021 |
| Otley Kids Club Limited | Education | 2017 |
| Otley Professional Limited | Professional & scientific | 2021 |
| Otley Road Convenience Ltd | Wholesale & retail | 2025 |
| Otley Road Convinience Store Ltd | Wholesale & retail | 2024 |
| Otley Road Flat Management Company Limited | Households | 1982 |
| Otleyfood Ltd | Hospitality | 2026 |
| OTM (UK) Limited | Professional & scientific | 2002 |
| OTM Industrial Roofing and Cladding Limited | Construction | 2020 |
| Otocare Audiology Ltd | Health & social care | 2024 |
| Otodoc Investments Limited | Financial services | 2026 |
| Otodoc Limited | Health & social care | 2018 |
| OTS Property Limited | Construction | 2016 |
| Otsneakerz Limited | Wholesale & retail | 2022 |
| Otter Consultancy Limited | Transport & logistics | 2025 |
| Otto Creative Limited | Professional & scientific | 2023 |
| Ottobars Ltd | Hospitality | 2021 |
| Ottoman Tufting Company Limited | Wholesale & retail | 2020 |
| Otty Holdings Ltd | Financial services | 2016 |
| Otty Sleep Ltd | Wholesale & retail | 2016 |
| OTU Property Solutions Ltd | Real estate | 2025 |
| Ouaddai/Bargo-Selleyhab Community-Uk(Obscuk) | Other services | 2009 |
| Ouais Bakery Ltd | Manufacturing | 2025 |
| Oualie Native Spirit RUM Ltd | Wholesale & retail | 2022 |
| Ouav Headingley Ltd | Wholesale & retail | 2025 |
| Oulton Care Limited | Hospitality | 2019 |
| Oulton Consulting Partners Limited | Professional & scientific | 2017 |
| Oulton Continuing Healthcare Consultancy Ltd | Health & social care | 2025 |
| Oulton Peritus Ltd | Professional & scientific | 2026 |
| Oulton Property Partners Limited | Real estate | 2025 |
| OUR Camp Ltd | Administrative & support | 2024 |
| OUR District Ltd | Hospitality | 2019 |
| OUR Farnley & Wortley CIC | Administrative & support | 2026 |
| OUR Future Scientist Ltd | Education | 2026 |
| OUR House Family Ltd | Administrative & support | 2021 |
| OUR Market Wigan Ltd | Wholesale & retail | 2024 |
| OUR Physio Group Limited | Health & social care | 2024 |
| Ourshelves Ltd | Information & communication | 2026 |
| OUT of Many People CIC | Education | 2024 |
| OUT of the Woods (Holbeck) Ltd | Hospitality | 2006 |
| Outco Surfacing Limited | Construction | 2008 |
| Outdoor Envy Landscapes and Construction Ltd | Administrative & support | 2021 |
| Outfront Ltd | Administrative & support | 2025 |
| Outlane Imaging Ltd | Health & social care | 2024 |
| Outlaws Hair Company Ltd | Other services | 2020 |
| Outlaws Yacht Club Ltd | Hospitality | 2012 |
| Outlet Mirrors and Furniture Limited | Wholesale & retail | 2017 |
| Outlier Group Ltd | Financial services | 2024 |
| Outnumbered Limited | Professional & scientific | 2015 |
| Outra Limited | Information & communication | 2016 |
| Outreach Care Solutions Limited | Administrative & support | 2020 |
| Outside of London Limited | Extraterritorial | 2015 |
| Outside the Box (Leeds) Ltd | Manufacturing | 2014 |
| Outsidein Developments 2 Limited | Professional & scientific | 2006 |
| Outskulpted Ltd | Arts & recreation | 2023 |
| Outsourcextra Ltd | Information & communication | 2025 |