| Octamos | Professional & scientific | 2026 |
| Octavia Chic Interiors Ltd | Professional & scientific | 2022 |
| Octo Demolition Limited | Construction | 2024 |
| Octopus Accounting Limited | Professional & scientific | 2017 |
| Octopus Accounts Limited | Professional & scientific | 2018 |
| Octopus Fostering Limited | Professional & scientific | 2021 |
| Octopus Outsourcing Limited | Administrative & support | 2022 |
| OD Barbershop Ltd | Other services | 2020 |
| Odin Engineering Services Ltd | Construction | 2023 |
| ODS Property Sourcing Ltd | Real estate | 2024 |
| Odss Ltd | Administrative & support | 2020 |
| Odunhair Collectionz Ltd | Other services | 2025 |
| Odyssey Coach Sales Ltd | Wholesale & retail | 2018 |
| Odyssey Renewables Ltd | Professional & scientific | 2026 |
| Oeco Services Limited | Agriculture & fishing | 2019 |
| OF Sharon & Carmel Ltd | Wholesale & retail | 2020 |
| Ofelia Ltd | Transport & logistics | 2025 |
| OFF Grid Systems Ltd | Professional & scientific | 2015 |
| Office Bay Limited | Construction | 2013 |
| Office Intelligence Ltd | Information & communication | 2025 |
| OG Classic UK Ltd | Wholesale & retail | 2021 |
| OG Italian Limited | Hospitality | 2022 |
| OG Padell Limited | Arts & recreation | 2026 |
| OGB Development Solutions Ltd | Construction | 2025 |
| Ogpd Ltd | Wholesale & retail | 2024 |
| Ohae Property Maintenance Limited | Construction | 2025 |
| OHL Solutions Limited | Construction | 2017 |
| OHM Electrical Services Limited | Construction | 2006 |
| OHT Ltd | Professional & scientific | 2025 |
| OJC Recruit Ltd | Professional & scientific | 2025 |
| Ojja Plumbing Solutions Ltd | Construction | 2025 |
| OJM Distributions Ltd | Wholesale & retail | 2026 |
| OLA247 Limited | Administrative & support | 2019 |
| Old Station Court Management Company Limited | Households | 2013 |
| Oldwoodscrew Ltd | Construction | 2024 |
| Olive Contracting Services Limited | Construction | 2016 |
| Olive Tree Caravan & Camping Park Ltd | Real estate | 2017 |
| Oliver Thomas Ltd | Hospitality | 2022 |
| Ollerton Plant & Civil Engineering Ltd | Construction | 2019 |
| Ollerton Properties Limited | Real estate | 2024 |
| Oltec Group Holding Ltd | Professional & scientific | 1991 |
| Oltec Group Trading Ltd | Administrative & support | 1953 |
| Oluashiru Ltd | Information & communication | 2019 |
| Oluwadamilola Crabbe Limited | Health & social care | 2025 |
| Olympia Coach Services Ltd | Extraterritorial | 2017 |
| Olympia Holdings Limited | Hospitality | 2022 |
| Olympia Travel UK Ltd | Transport & logistics | 2002 |
| Omega Counselling Ltd | Health & social care | 2018 |
| Omega Roofing Ltd | Construction | 2009 |
| Omni Personnel Limited | Administrative & support | 2024 |
| Omniplas Ltd | Wholesale & retail | 2018 |
| Omnisky Ltd | Manufacturing | 2025 |
| ON the Baize Limited | Information & communication | 2026 |
| ON the Board Solutions Ltd | Professional & scientific | 2022 |
| ON the Way Coffee Shop CIC | Hospitality | 2025 |
| ON Track Tees Ltd | Wholesale & retail | 2024 |
| ON Your Services BIZ Limited | Manufacturing | 2003 |
| Oncall GAS and Plumbing Services Ltd | Construction | 2013 |
| One Care 4 YOU Ltd | Health & social care | 2024 |
| One Free MAN Ltd | Information & communication | 2017 |
| One Leisure Birmingham Ltd | Hospitality | 2023 |
| One Leisure Liverpool Ltd | Hospitality | 2023 |
| One Leisure Manchester Ltd | Hospitality | 2023 |
| One Stop Grocery Limited | Wholesale & retail | 2022 |
| One System Software Ltd | Information & communication | 2026 |
| One TEN Deli Limited | Hospitality | 2025 |
| One Together C.I.C. | Education | 2019 |
| One Vision Group Wigan Ltd | Hospitality | 2021 |
| Onitsarse Limited | Manufacturing | 2025 |
| Online Bootroom Ltd | Wholesale & retail | 2020 |
| Ontime Refurbishment Specialists Ltd | Construction | 2016 |
| Onyx Ling Ltd | Wholesale & retail | 2019 |
| Opej UK Limited | Extraterritorial | 2024 |
| Open Access Services Ltd. | Construction | 2018 |
| Operator AI UK Ltd | Information & communication | 2026 |
| Operator and Driver Support Services Ltd | Administrative & support | 2020 |
| Opti-Lease Limited | Wholesale & retail | 2016 |
| Optical Ventures Ltd | Professional & scientific | 2022 |
| Optima Lettings Agency Ltd | Real estate | 2025 |
| Optimise Supply Limited | Professional & scientific | 2025 |
| Optimum Contract Hire Limited | Administrative & support | 2010 |
| Optimum Design (UK) Limited | Wholesale & retail | 1997 |
| Opus Digital Marketing Ltd | Information & communication | 2025 |
| OR7 Limited | Arts & recreation | 2025 |
| Oracle Home Solutions Limited | Professional & scientific | 2010 |
| Orange Track Telematics Ltd | Professional & scientific | 2016 |
| ORB Rope Access Limited | Construction | 2016 |
| Orbitz Business Solutions Ltd | Information & communication | 2020 |
| Orbivos | Professional & scientific | 2025 |
| Orcass Limited | Information & communication | 2020 |
| Orchid Commercial Consultants Limited | Other services | 2018 |
| Orchid House Emporium Ltd | Hospitality | 2017 |
| Orderpower Limited | Real estate | 1980 |
| Organic Secrets UK Ltd | Wholesale & retail | 2019 |
| Orgon Financial Services PLC | Professional & scientific | 2017 |
| Orinsis Ltd | Information & communication | 2021 |
| Orionson | Professional & scientific | 2026 |
| Orium Ltd | Professional & scientific | 2025 |
| Ormeshers Tyres Limited | Wholesale & retail | 2014 |
| Orre Limited | Wholesale & retail | 2022 |