| Onkar Agile Ltd | Information & communication | 2016 |
| Onlii Holdings Limited | Financial services | 2026 |
| Online Car and Van Leasing Ltd. | Administrative & support | 2017 |
| Online Commerce 2 U Ltd | Professional & scientific | 2026 |
| Online Groceries Ltd | Wholesale & retail | 2020 |
| Online Leisure Systems Limited | Information & communication | 1991 |
| Online Quick Service Ltd | Information & communication | 2024 |
| Online Taxation Ltd | Professional & scientific | 2020 |
| ONLINE4YOU Limited | Wholesale & retail | 2016 |
| Onlinefootballcourses.Com Ltd | Extraterritorial | 2025 |
| Onlinetech Ltd | Wholesale & retail | 2026 |
| Onlux Ltd | Administrative & support | 2023 |
| Only Best Limited | Administrative & support | 2021 |
| Only Fanz Maintenance Ltd | Professional & scientific | 2023 |
| Only Ice Ltd | Hospitality | 2024 |
| Onlypaws Grooming Leeds Limited | Agriculture & fishing | 2024 |
| Onoma Ltd | Health & social care | 2016 |
| Onomo Group CIC | Construction | 2025 |
| Onpoint Traders Ltd | Wholesale & retail | 2024 |
| Onpointq Ltd | Wholesale & retail | 2025 |
| ONS Security Solutions Limited | Financial services | 2019 |
| Onsite (Commercial) Limited | Construction | 2020 |
| Onsite Fleet Solutions Ltd | Wholesale & retail | 2023 |
| Onsitenow Website Design Limited | Information & communication | 2009 |
| Onstate Limited | Professional & scientific | 2010 |
| Ontario Inspirations Limited | Administrative & support | 2009 |
| Onto the Page Limited | Administrative & support | 2008 |
| Ontrac Limited | Information & communication | 2008 |
| Ontrac Technology Limited | Financial services | 2013 |
| Ontrack Consultancy Services Limited | Professional & scientific | 2016 |
| Ontrack Developments Limited | Real estate | 2024 |
| Onwara Movement Ltd | Wholesale & retail | 2026 |
| Onwards & Upwards Property Ltd | Real estate | 2010 |
| Onyou Investments Ltd | Administrative & support | 2017 |
| Onyu Investing Limited | Financial services | 2024 |
| Onyva Studio Ltd | Professional & scientific | 2024 |
| Onyx Business Consulting Limited | Administrative & support | 2013 |
| Onyx Cars Ltd | Wholesale & retail | 2011 |
| Onyx Concept Limited | Wholesale & retail | 2025 |
| Onyx Properties (Yorkshire) Limited | Construction | 2019 |
| Onyx Recycling Ltd | Water & waste | 2017 |
| Onyxcloud Ltd | Information & communication | 2026 |
| Onzi Ltd | Transport & logistics | 2026 |
| Oobikit Ltd | Manufacturing | 2017 |
| OOH LA LAA Leeds Ltd | Hospitality | 2024 |
| OOJ Homecare Services Ltd | Other services | 2015 |
| OOJ Properties Ltd | Real estate | 2024 |
| Ookushana Ltd | Health & social care | 2016 |
| Oosh Ltd | Professional & scientific | 2023 |
| OP Commercial Ltd | Financial services | 2016 |
| OP8 Logistics Limited | Transport & logistics | 2019 |
| OPA Catering Ltd | Wholesale & retail | 2019 |
| Opad Limited | Wholesale & retail | 2025 |
| Opal Acoustics Ltd | Professional & scientific | 2026 |
| Opal Leeds Limited | Real estate | 2021 |
| Opal Software Associates Limited | Information & communication | 2021 |
| OPB Logistics Limited | Transport & logistics | 2023 |
| OPE Consulting Services Ltd | Education | 2021 |
| Open Door Financial Planning Limited | Professional & scientific | 2013 |
| Open Introvert Ltd | Professional & scientific | 2019 |
| Open Path Agency Limited | Information & communication | 2012 |
| Open Road Software Limited | Information & communication | 2011 |
| Open Source Arts Ltd | Arts & recreation | 2016 |
| Open Table Strategies Limited | Professional & scientific | 2025 |
| Open View Therapy Limited | Health & social care | 2025 |
| Open White Design Ltd | Professional & scientific | 2012 |
| Open World Analytics Ltd | Professional & scientific | 2022 |
| Openbook Telecoms Ltd | Information & communication | 2025 |
| Openehr UK Foundation CIC | Information & communication | 2024 |
| Opening Systems Limited | Construction | 2010 |
| Openit Solutions Limited | Information & communication | 2011 |
| Openix Limited | Extraterritorial | 2024 |
| Openkey Ltd | Real estate | 2025 |
| Openkey Property Ltd | Construction | 2026 |
| Openminimarket Ltd | Wholesale & retail | 2024 |
| Opennerz Technologies Limited | Information & communication | 2025 |
| Openpsm Limited | Information & communication | 2022 |
| Openspace Leeds Limited C.I.C. | Other services | 2009 |
| Opentech Limited | Information & communication | 1986 |
| Opera Enterprises Ltd | Administrative & support | 2025 |
| Opera North Limited | Arts & recreation | 1981 |
| Opera North Trading Limited | Hospitality | 2014 |
| Operation Platform One Limited | Information & communication | 2023 |
| Operational Escort Services Ltd | Administrative & support | 2025 |
| Operational Excellence Solutions Ltd | Professional & scientific | 2025 |
| Operational Technology Integrators Limited | Information & communication | 2022 |
| Operdi Limited | Information & communication | 2024 |
| Opes Preservation Limited | Administrative & support | 2016 |
| Opex Coaching Ltd | Professional & scientific | 2025 |
| Opex Notion Ltd | Information & communication | 2018 |
| OPH Investco Limited | Real estate | 2023 |
| OPH Northern Limited | Hospitality | 2025 |
| OPH NW Ltd | Real estate | 2022 |
| OPH Property Holdings Limited | Hospitality | 2009 |
| OPH Telford Limited | Real estate | 2019 |
| Opincans Retrofit Ltd | Professional & scientific | 2025 |
| Opium Beauty Limited | Other services | 2016 |
| Opium Beauty Spa Limited | Other services | 2016 |
| OPM (Labels & Packaging) Group Limited | Manufacturing | 1972 |
| OPM Advance Limited | Manufacturing | 2016 |