| Optimum Consultancy Group Ltd | Professional & scientific | 2025 |
| Optimum Consulting Services Limited | Information & communication | 2021 |
| Optimum Healthcare NB Limited | Health & social care | 2023 |
| Optimum Performance Coaching Ltd | Professional & scientific | 2023 |
| Optimum Sports Consultants Ltd | Other services | 2024 |
| Optimus Construction Services Ltd | Construction | 2023 |
| Optimus Hire Ltd | Administrative & support | 2018 |
| Optimus Innovations Limited | Real estate | 2017 |
| Optipure Ltd | Manufacturing | 2024 |
| Opto Luton Limited | Hospitality | 2018 |
| Optom 360 Ltd | Health & social care | 2021 |
| Optumus Consulting Limited | Information & communication | 2024 |
| Opulence Engineering Ltd | Manufacturing | 2022 |
| Opulent Glow Limited | Other services | 2025 |
| Opulent IT Solutions Ltd | Information & communication | 2014 |
| Opulent Suites Ltd | Hospitality | 2024 |
| Opus Crua Ltd | Health & social care | 2021 |
| Opus Investments Limited | Real estate | 2023 |
| Opus Meum Limited | Education | 2016 |
| Oqab Travel Ltd | Administrative & support | 2025 |
| OR Innovation Limited | Professional & scientific | 2017 |
| ORA Management Limited | Information & communication | 2014 |
| Oraanj Construction Limited | Construction | 2025 |
| Oraanj Digital Marketing Limited | Manufacturing | 2025 |
| Oraanj Interior Design Limited | Professional & scientific | 2023 |
| Oraanj Limited | Construction | 2018 |
| Oracle & Oracle Housing Association CIC | Real estate | 2025 |
| Oracle Flooring Ltd | Construction | 2022 |
| Oraconsultancy Professional DB Services Limited | Information & communication | 2014 |
| Oraexcel Capital Ltd | Financial services | 2025 |
| Orakzai CNG Filling Station UK Ltd | Wholesale & retail | 2024 |
| Oral Connect Ltd | Information & communication | 2025 |
| Oral Facial Skin Surgery Ltd | Health & social care | 2018 |
| Oram Digital Marketing Limited | Professional & scientific | 2022 |
| Oram Place Associates Limited | Real estate | 2009 |
| Orange Canyon Limited | Hospitality | 2017 |
| Orange Cloud Bidco Limited | Health & social care | 2020 |
| Orange Cloud Midco Limited | Health & social care | 2020 |
| Orange Cloud Topco Limited | Health & social care | 2020 |
| Orange Fisher Limited | Other services | 2016 |
| Orange Fisher W7 Ltd | Other services | 2018 |
| Orange Jupiter Limited | Financial services | 2024 |
| Orange Private Limited | Health & social care | 2018 |
| Orange Salon Limited | Other services | 2025 |
| Orange Sky Properties Limited | Real estate | 2021 |
| Orange Star Properties Ltd | Real estate | 2022 |
| Orange Times Limited | Construction | 2021 |
| Orangeey Productions Limited | Professional & scientific | 2019 |
| Orangehat Limited | Construction | 2025 |
| Orantix Ltd | Real estate | 2025 |
| ORB Optical Ltd | Wholesale & retail | 2011 |
| Orbis Consumer Products Limited | Real estate | 1998 |
| Orbis Property Investments Limited | Real estate | 2008 |
| Orbit Immigration Ltd | Other services | 2020 |
| Orbit PR Limited | Professional & scientific | 2005 |
| Orbit Services Private Limited | Professional & scientific | 2014 |
| Orbital Investment Limited | Real estate | 2023 |
| Orbital Student Holdings Limited | Construction | 2024 |
| Orbitradex Ltd | Wholesale & retail | 2023 |
| Orbitronyx Consultants Ltd | Professional & scientific | 2019 |
| Orbrick Consulting Limited | Information & communication | 2024 |
| Orchadia Homes Ltd | Real estate | 2025 |
| Orchard Healthcare Limited | Health & social care | 2015 |
| Orchard Street Properties Limited | Real estate | 2022 |
| Orchard Theatre Limited | Arts & recreation | 2008 |
| Orchid Haus Ltd | Wholesale & retail | 2020 |
| Orchid International Technologies Limited | Information & communication | 2023 |
| Orchid Psychology Ltd | Education | 2021 |
| Orchid Speech Therapy Ltd | Health & social care | 2022 |
| Orchid Student Development Limited | Construction | 2025 |
| Orchid Student Holdings Limited | Financial services | 2025 |
| Orchid Student Living Limited | Real estate | 2025 |
| Orchid Vase Films Limited | Information & communication | 2022 |
| Ordnance London Ltd | Other services | 2019 |
| Orel Express Limited | Construction | 2016 |
| Oretha and Sons Limited | Health & social care | 2023 |
| Organic Fresh Limited | Wholesale & retail | 2016 |
| Organic Heating Ltd | Construction | 2017 |
| Organised Little Living Limited | Professional & scientific | 2025 |
| Orgatex UK Limited | Administrative & support | 2011 |
| Orient Realty Limited | Real estate | 2025 |
| Orient Retail Airdrie Ltd | Wholesale & retail | 2024 |
| Orient Retail Ltd | Wholesale & retail | 2024 |
| Oriental Press (UK) Limited | Manufacturing | 1993 |
| Original Alan Solomons Insurance Services | | 1986 |
| Original Blues Clothing Company Ltd. | Manufacturing | 2000 |
| Original Real Estate Limited | Hospitality | 2017 |
| Orion Accounting Ltd | Extraterritorial | 2013 |
| Orion Demolition Limited | Water & waste | 2019 |
| Orion Electrical Installations Ltd | Construction | 2010 |
| Orion Electrical Services Ltd | Construction | 2023 |
| Orion Electricals Limited | Construction | 1993 |
| Orion Investment Properties Ltd | Real estate | 2023 |
| Orion Management Ltd | Households | 2020 |
| Orion TH Ltd | Real estate | 2025 |
| Orisco Limited | Wholesale & retail | 2024 |
| Orito Building Ltd | Construction | 2014 |
| Orium Construction Ltd | Construction | 2011 |
| Orivale Ltd | Wholesale & retail | 2025 |
| Oriya Ltd | Professional & scientific | 2015 |