| Onemkh Limited | Real estate | 2020 |
| Onesoft Accounting Ltd | Professional & scientific | 2017 |
| Oneten Talent Solutions Ltd | Professional & scientific | 2019 |
| Onevision Operation Ltd | Professional & scientific | 2026 |
| Onit Europe Ltd | Information & communication | 2019 |
| Onkaar Support Ltd | Health & social care | 2021 |
| Onkar Builders Ltd | Construction | 2018 |
| Onkar Ltd | Transport & logistics | 2020 |
| Onkar Properties Investor Ltd | Real estate | 2023 |
| Online Haven Limited | Wholesale & retail | 2023 |
| Online Trade Wizard Limited | Wholesale & retail | 2020 |
| Online Transport Training Ltd | Administrative & support | 2022 |
| Online UK Tax Returns Limited | Professional & scientific | 2023 |
| Only Special Offers Limited | Information & communication | 2020 |
| Onlyfabs Welding Limited | Manufacturing | 2026 |
| Onmt Slough Ltd | Transport & logistics | 2024 |
| ONN Services Ltd | Administrative & support | 2023 |
| ONSITE365SOLUTIONS Ltd | Transport & logistics | 2019 |
| Ontour Logistic Service UK Limited | Transport & logistics | 2012 |
| Ontra Healthcare Ltd | Health & social care | 2019 |
| Onua Limited | Wholesale & retail | 2017 |
| Onwards and Forwards Ltd | Administrative & support | 2026 |
| Onyx Corp Ltd | Construction | 2025 |
| Onyx Performance Ltd | Wholesale & retail | 2020 |
| Onyxo Real Estate Ltd | Real estate | 2021 |
| OP Cleaning Services Ltd | Administrative & support | 2025 |
| OP Security Services Ltd | Administrative & support | 2022 |
| Opalrise Roofing Limited | Construction | 2024 |
| Opals Funerals Ltd | Other services | 2014 |
| Opdenergy UK 10 Limited | Energy | 2023 |
| Opdenergy UK 11 Limited | Energy | 2023 |
| Opdenergy UK 12 Limited | Energy | 2024 |
| Opdenergy UK 13 Limited | Energy | 2024 |
| Opdenergy UK 14 Limited | Energy | 2024 |
| Opdenergy UK 15 Limited | Energy | 2024 |
| Opdenergy UK 16 Limited | Energy | 2024 |
| Opdenergy UK 17 Limited | Energy | 2024 |
| Opdenergy UK 18 Limited | Energy | 2024 |
| Opdenergy UK 19 Limited | Energy | 2024 |
| Opdenergy UK 20 Limited | Energy | 2024 |
| Opdenergy UK 21 Limited | Energy | 2024 |
| Opdenergy UK 22 Limited | Energy | 2024 |
| Opdenergy UK 23 Limited | Energy | 2024 |
| Opdenergy UK 9 Limited | Energy | 2023 |
| Open Arms & Partners CIC | Health & social care | 2026 |
| Open Climb Aviation Ltd | Transport & logistics | 2018 |
| Open Mind Corporate Ltd | Financial services | 2025 |
| Open Mind Investments (UK) Ltd | Real estate | 2024 |
| Open Storage Yards Ltd | Real estate | 2024 |
| Open Valley Roofing Ltd | Construction | 2025 |
| OPM Homes Ltd | Real estate | 2025 |
| Oprify Trading Ltd | Wholesale & retail | 2026 |
| Optane Networks Ltd | Information & communication | 2021 |
| Optec Consultancy Ltd | Information & communication | 2016 |
| Optec Properties Limited | Real estate | 2020 |
| Opti Plus Services Ltd | Water & waste | 2011 |
| Optigate Ltd | Wholesale & retail | 2025 |
| Optima Law Solicitors Limited | Professional & scientific | 2017 |
| Optimal Motion Ltd | Education | 2020 |
| Optimise Analytics Ltd | Information & communication | 2020 |
| Optimum Consultancy Limited | Professional & scientific | 2019 |
| Optimum Investment Holdings Limited | Financial services | 2018 |
| Optimum Power Ltd | Arts & recreation | 2024 |
| Optimum Tech Holdings Limited | Wholesale & retail | 2023 |
| Optimus Infotech Ltd | Wholesale & retail | 2025 |
| Optivus MSP Ltd. | Information & communication | 2025 |
| Optomeyes Care Limited | Health & social care | 2014 |
| Opulence Holdings Limited | Real estate | 2018 |
| Opulence Wealth & Investments Finance Limited | Financial services | 2025 |
| Opulent Attire Ltd | Wholesale & retail | 2023 |
| Opulent Collection Ltd | Wholesale & retail | 2015 |
| Opulent Property Services Ltd | Construction | 2024 |
| Opulent Residences Ltd | Households | 2022 |
| Oracle Care Group Ltd | Administrative & support | 2019 |
| Oracle Healthcare Recruitment Limited | Health & social care | 2015 |
| Oracle Private Hire Ltd | Transport & logistics | 2025 |
| Oral Creation Orthodontics Limited | Wholesale & retail | 2020 |
| Oralsmile Ltd | Health & social care | 2025 |
| Orange Business Holdings UK Limited | Administrative & support | 1995 |
| Orange Business UK Limited | Administrative & support | 1991 |
| Orange Globe Ltd | Information & communication | 2017 |
| Orange Towers Limited | Wholesale & retail | 2017 |
| Orangestill Limited | Professional & scientific | 2025 |
| Orbit Cargo Services Ltd | Transport & logistics | 2013 |
| Orbit Homes Ltd | Real estate | 2025 |
| Orbits Business Solutions Limited | Information & communication | 2010 |
| Orbiway Limited | Wholesale & retail | 2026 |
| Orchard Estates Associates Ltd | Real estate | 2023 |
| Orchard Mill Limited | Financial services | 2026 |
| Orchardville Consulting Ltd | Manufacturing | 2017 |
| Orchardville Estates Ltd | Real estate | 2021 |
| Orchid Software Limited | Information & communication | 2004 |
| Orcus Developments Ltd | Professional & scientific | 2015 |
| Ordae Ltd | Wholesale & retail | 2025 |
| Order Carry Ltd | Wholesale & retail | 2023 |
| Ordergrade Limited | | 1974 |
| Organicap Ltd | Other services | 2022 |
| Organisational Diagnostics Limited | Education | 2024 |
| Orgavit Ltd | Other services | 2024 |
| Orgo Drinks Ltd | Manufacturing | 2023 |