| Opus Credit Partners Limited | Professional & scientific | 2025 |
| Opus Equity Limited | Professional & scientific | 2014 |
| Opus Grant Services Limited | Professional & scientific | 2026 |
| Opus Hyde House LLP | | 2007 |
| Opus Payment Solutions UK Limited | Information & communication | 2018 |
| Opus Pear Tree Limited | Professional & scientific | 2017 |
| Opus Restructuring LLP | | 2013 |
| Opus Strategic Advisory Limited | Professional & scientific | 2023 |
| Opus Support Limited | Information & communication | 2022 |
| Opus Workforce Ltd | Professional & scientific | 2017 |
| Opusdata Ltd | Information & communication | 2018 |
| Oqoud Ltd | Administrative & support | 2020 |
| OR Forklift Training Ltd | Professional & scientific | 2025 |
| Oracy Education Ltd | Education | 2024 |
| Orange HUT Solutions Limited | Information & communication | 2007 |
| Orbitconnect Ltd | Information & communication | 2025 |
| Orchard Lodge Properties (MK) Limited | Real estate | 2017 |
| Orchard Marsh Electrical Ltd | Construction | 2019 |
| Orchard Melamine Products Limited | Professional & scientific | 2001 |
| Orchard Printing Limited | Manufacturing | 2018 |
| Orchards Investments Limited | Real estate | 2019 |
| Orchards Investments SPV1 Ltd | Real estate | 2024 |
| Orchid Beauty Agency Ltd | Administrative & support | 2016 |
| Orchid Industrial Solutions Limited | Information & communication | 2000 |
| Orchids and Cakes Ltd | Manufacturing | 2023 |
| Oregbefield Limited | Construction | 2018 |
| Oreko Health and Wellness Limited | Health & social care | 2022 |
| Oretek Limited | Professional & scientific | 2016 |
| Orford House Limited | Real estate | 2008 |
| Organic CBD Ltd | Agriculture & fishing | 2025 |
| Organic Foods, Drinks & Investments Holdings Ltd | Financial services | 2002 |
| Organic Souk Hub Ltd | Wholesale & retail | 2023 |
| Organization for Environmental Research and Green Innovation (Oergi) | Professional & scientific | 2022 |
| Oriac Ltd | Information & communication | 2024 |
| Oriel Technology Limited | Professional & scientific | 2012 |
| Orien UK Ltd | Wholesale & retail | 2008 |
| Orient Capital Ltd | Real estate | 2017 |
| Orient Residential Ltd | Real estate | 2026 |
| Orient Rods UK Limited | Transport & logistics | 2020 |
| Orienta Advisory Limited | Professional & scientific | 2026 |
| Oriental Joyful Co., Ltd. | Wholesale & retail | 2023 |
| Orientate Ltd | Professional & scientific | 2019 |
| Oriflame Cosmetics UK Limited | Administrative & support | 2022 |
| Origami Wings CIC | Education | 2025 |
| Origin Ghar Limited | Real estate | 2021 |
| Origin Impact Ltd | Education | 2026 |
| Origin Rental Limited | Real estate | 2023 |
| Origin Soft Ltd | Information & communication | 2010 |
| Origin Stays Limited | Real estate | 2026 |
| Origincreate Limited | Wholesale & retail | 2025 |
| Origindc Ltd | Professional & scientific | 2016 |
| Origo Coaching Ltd | Professional & scientific | 2025 |
| Orion Properties UK Limited | Real estate | 2000 |
| Orion Security Consulting Limited | Information & communication | 2022 |
| Orion Wholesale Ltd | Wholesale & retail | 2019 |
| Orionpath Ltd | Wholesale & retail | 2023 |
| Oriva Retail Ltd | Wholesale & retail | 2026 |
| Orizont Limited | Transport & logistics | 2018 |
| Orjona Hair and Beauty Ltd | Other services | 2018 |
| Orkinox Limited | Construction | 2016 |
| Ormscorp Limited | Hospitality | 2024 |
| Ormshomes Ltd | Real estate | 2024 |
| Orpheus Corporate Finance LLP | | 2008 |
| Ortac International Limited | Wholesale & retail | 2020 |
| Orthodontic Innovations and Solutions Limited | Health & social care | 2016 |
| Orthodox Vision Media Ltd | Information & communication | 2025 |
| Ortiz Complete Contractors Ltd | Construction | 2023 |
| Oruen Rehab Ltd | Health & social care | 2019 |
| Osai-Uk Ltd | Manufacturing | 1997 |
| Osakwe-Mezue Press Ltd | Information & communication | 2021 |
| Oscabe Ltd | Professional & scientific | 2024 |
| Oscar's Cocktail Box Limited | Hospitality | 2019 |
| Osco Limited | Professional & scientific | 1995 |
| OSD Limited | Extraterritorial | 2019 |
| OSE Oifoh Consultancy Ltd | Information & communication | 2025 |
| Oseitutu Limited | Health & social care | 2026 |
| Oselli Limited | Wholesale & retail | 1976 |
| Osewelle Software Solutions Limited | Information & communication | 2015 |
| OSH Qualifications Board UK Ltd | Education | 2023 |
| Osie Holdings Limited | Arts & recreation | 2023 |
| Osietv Limited | Arts & recreation | 2023 |
| Oske Equestrian Fitness Limited | Arts & recreation | 2026 |
| Osman Ltd | Real estate | 2008 |
| Osmium Electrical Ltd | Construction | 2025 |
| Osob Limited | Manufacturing | 2016 |
| Ossmc Limited | Professional & scientific | 2017 |
| Ostara Systems Ltd | Information & communication | 2003 |
| Osteo Mission Limited | Other services | 2020 |
| OT Properties Ltd | Real estate | 2022 |
| OT Systems Limited | Information & communication | 2011 |
| Otanka&Jessie Ltd | Health & social care | 2021 |
| Oteng Agyei Transport Ltd | Transport & logistics | 2013 |
| Otidelis Investments Ltd | Hospitality | 2021 |
| Otif-Zd Logistics Limited | Wholesale & retail | 2016 |
| Otilia Ro-Market Limited | Wholesale & retail | 2026 |
| Otis Medicals Limited | Health & social care | 2012 |
| OTO International Limited | Wholesale & retail | 2018 |
| OTO Prime Properties Limited | Real estate | 2026 |
| Otobrothers Ltd | Information & communication | 2026 |
| Otofill Ltd | Information & communication | 2026 |