| Oakweald Publishing Limited | Information & communication | 2017 |
| Oana Spedition Ltd | Transport & logistics | 2020 |
| Oasis Cars Ltd | Wholesale & retail | 2024 |
| Oasis Web Services Ltd | Information & communication | 2025 |
| OBA Healthcare Limited | Health & social care | 2015 |
| Obelix Ltd | Construction | 2025 |
| OC Hort Limited | Administrative & support | 2024 |
| Occasianz Ltd | Arts & recreation | 2004 |
| Ocean & Architectural Models Ltd. | Manufacturing | 2004 |
| Oceanwave Analytics & Consulting Services Limited | Professional & scientific | 2023 |
| Octavian ALI Ltd | Transport & logistics | 2015 |
| OCW Investments Limited | Real estate | 2024 |
| OD Racing Ltd | Arts & recreation | 2025 |
| ODC Properties Ltd | Real estate | 2026 |
| Odisha Samaj of the UK Ltd | Health & social care | 2024 |
| Odisha Society of United Kingdom (Osuk) C.I.C. | Health & social care | 2023 |
| ODM Services Limited | Construction | 2014 |
| Odogun Property Management Ltd | Real estate | 2024 |
| Odohia Ltd | Administrative & support | 2020 |
| Odum Search Limited | Professional & scientific | 2017 |
| Odwyer Consulting Limited | Professional & scientific | 2026 |
| Oesophago Ltd | Manufacturing | 2018 |
| Office Management Limited | Administrative & support | 2010 |
| OFG Flooring Ltd | Construction | 2022 |
| Ogbeni Mmxii Ltd | Professional & scientific | 2026 |
| Ogilba Ltd | Transport & logistics | 2025 |
| OH Estates Private Limited | Real estate | 2020 |
| OH Healthcare Private Limited | Administrative & support | 2025 |
| OH! Equine Maintenance Ltd | Extraterritorial | 2024 |
| Ohare Inspections Ltd | Professional & scientific | 2025 |
| Ohitar Consultancy Ltd | Administrative & support | 2018 |
| OK2006 Ltd | Real estate | 2024 |
| Okedejapah Trauma Victims Initiative CIC | Other services | 2026 |
| Okez Drylining Limited | Construction | 2019 |
| Olaiya Foods UK Ltd | Wholesale & retail | 2025 |
| Olan Limited | Wholesale & retail | 2023 |
| Olantek Limited | Information & communication | 2018 |
| Oldshoreham Ltd | Wholesale & retail | 2026 |
| Oleh Estates Limited | Administrative & support | 2024 |
| Olifi Ltd | Manufacturing | 2016 |
| Olighton Limited | Professional & scientific | 2020 |
| Olison Properties Ltd | Real estate | 2024 |
| Olisus Beauty Home Ltd | Administrative & support | 2018 |
| Olives Consultancy Services Ltd | Professional & scientific | 2014 |
| Olivet Insights CIC | Education | 2025 |
| Oluola and Co Limited | Health & social care | 2024 |
| OM Buildings and Construction Limited | Real estate | 2024 |
| OMD & Justin Ltd | Manufacturing | 2024 |
| Omega-Hope Healthcare Limited | Health & social care | 2015 |
| Omer Removals Ltd | Transport & logistics | 2026 |
| Omfal Consulting Limited | Information & communication | 2021 |
| Omix Construction Ltd | Construction | 2020 |
| OMO OLU Enterprises Ltd | Information & communication | 2022 |
| OMZ Cods Ltd | Hospitality | 2026 |
| OMZ Properties Limited | Real estate | 2026 |
| ON Site Services (Gravesend) Limited | Manufacturing | 1986 |
| ON the Spot Printing Service Ltd | Manufacturing | 2025 |
| ON Time Transport Limited | Transport & logistics | 2022 |
| ON Top Pest Control Ltd | Administrative & support | 2026 |
| ON Works Ltd. | Information & communication | 2012 |
| Onaivstone Ltd | Information & communication | 2021 |
| Onay Ltd | Other services | 2019 |
| One Call Cleaning Services Ltd | Administrative & support | 2024 |
| One Call Construction Solutions Limited | Administrative & support | 2020 |
| One Call Contractors Ltd | Construction | 2011 |
| One Cards & Stationery Ltd | Other services | 2017 |
| One Damson Tree Accountants Ltd | Professional & scientific | 2023 |
| One Stanmer Street Limited | Real estate | 1995 |
| One Way Clearance Ltd | Water & waste | 2024 |
| One World Consulting (UK) Ltd | Professional & scientific | 2018 |
| Onepulse PAY Ltd | Financial services | 2024 |
| Oneupclean Ltd | Administrative & support | 2016 |
| Onkar Logistics Ltd | Transport & logistics | 2025 |
| Online Store Ltd | Wholesale & retail | 2019 |
| Onshelf Retail Ltd | Other services | 2015 |
| Ontime Accountancy Ltd | Administrative & support | 2021 |
| Ontime Accountancy Practice Ltd | Professional & scientific | 2021 |
| Ontime Construction Ltd | Construction | 2025 |
| ONU Euro Logistics Ltd | Transport & logistics | 2010 |
| ONY Transport Ltd | Transport & logistics | 2020 |
| Onyemax Property Ltd | Real estate | 2025 |
| Onyx Maintenance Ltd | Construction | 2023 |
| Oparin Transport Ltd | Transport & logistics | 2016 |
| Open Range Development Limited | Information & communication | 2026 |
| Operational Security UK- Security & Private Investigation Ltd | Other services | 2022 |
| Opportunities With Experience - O-W-E - C.I.C. | Education | 2013 |
| Optimum Construction & Renovations Ltd | Administrative & support | 2020 |
| OPTIMUM4 Investments Ltd | Financial services | 2023 |
| Optimus Access Limited | Construction | 2011 |
| Optimus Construction Ltd | Construction | 2015 |
| Option Services (UK) Limited | Administrative & support | 2015 |
| Opulent Aesthetics RT Ltd | Health & social care | 2019 |
| Opus Solution Limited | Administrative & support | 2023 |
| Orange Estate Agents Limited | Real estate | 2013 |
| Orange News Limited | Wholesale & retail | 2019 |
| Orange Property Services (UK) Ltd | Real estate | 2008 |
| ORB Trading and Holdings Ltd | Financial services | 2025 |
| ORB VR Limited | Arts & recreation | 2022 |
| Orchard View Stores Limited | Wholesale & retail | 2026 |
| Ordrup Ltd | Information & communication | 2026 |