| OGO Clothing Ltd | Manufacturing | 2025 |
| Ogrady Data Solutions Limited | Information & communication | 2007 |
| OH SO Sweet Grazing and Event Hire Ltd | Hospitality | 2023 |
| OH YES1 Limited | Hospitality | 2019 |
| Ohana Education Ltd | Education | 2022 |
| Ojwh Ltd | Wholesale & retail | 2023 |
| Okami Media Limited | Professional & scientific | 2018 |
| Okepalm Limited | | 1989 |
| Okik Ltd | Wholesale & retail | 2021 |
| Okmed Limited | Real estate | 2025 |
| Okpb Limited | Arts & recreation | 2012 |
| OLB Logistics Ltd | Transport & logistics | 2026 |
| Old English Builders Ltd | Construction | 2025 |
| Old Hall Events LLP | | 2022 |
| Old Mill Memorials Ltd | Manufacturing | 2026 |
| Old NR 6 Ltd | Wholesale & retail | 2025 |
| Old Oak Craft Ltd | Wholesale & retail | 2026 |
| Old People Welfare Society | Health & social care | 2017 |
| Old Road Gardens Limited | Construction | 2026 |
| Old Star Farm Ltd | Extraterritorial | 2021 |
| Old Tramway Holdings Ltd | Real estate | 2023 |
| Oldacre Properties Limited | Real estate | 2020 |
| Oldham Court Management Company Limited | Real estate | 2024 |
| Oldham Engineering Group Training Association Limited (the) | Education | 1970 |
| Olekso Universal Ltd | Health & social care | 2020 |
| OLI Holdings Limited | Financial services | 2020 |
| Oliash Ltd | Health & social care | 2026 |
| Olive Italian Ltd | Hospitality | 2023 |
| Oliver Burley Medical Devices Limited | Manufacturing | 2023 |
| Oliver Takeaways Ltd | Hospitality | 2024 |
| Olivetree Solutions Ltd | Wholesale & retail | 2022 |
| Olivia Arts Studio Ltd | Wholesale & retail | 2025 |
| Olivia James Cosmetics Ltd | Wholesale & retail | 2020 |
| Olivia Towns PUB Company Limited | Hospitality | 2018 |
| Ollie Barbers Ltd | Other services | 2023 |
| Olmo House Limited | Real estate | 2013 |
| Olttrans Limited | Transport & logistics | 2019 |
| Olympic of Waterhouses Limited | Construction | 1979 |
| Omar Qayum Ltd | Extraterritorial | 2024 |
| Omena Creative Ltd | Wholesale & retail | 2022 |
| Omer Property Investments Ltd | Real estate | 2023 |
| Omer UK Mobile Ltd | Wholesale & retail | 2023 |
| Omerit Limited | Wholesale & retail | 2025 |
| OMG Clearance Stock UK Limited | Wholesale & retail | 2026 |
| OMJ Developments Limited | Construction | 2003 |
| Omnaturique Ltd | Wholesale & retail | 2025 |
| Omni Asset Group Limited | Financial services | 2026 |
| Omni Lift Services Ltd | Manufacturing | 2022 |
| Omnika Ltd | Wholesale & retail | 2025 |
| Omnipay Ltd | Professional & scientific | 2021 |
| Omniscient Insights Ltd | Health & social care | 2023 |
| Omniverse Limited | Wholesale & retail | 2014 |
| Omotayo Construction Services Ltd | Construction | 2025 |
| Omree Ltd | Other services | 2019 |
| Omtec Limited | Professional & scientific | 2023 |
| ON the Horizon Counselling Services CIC | Education | 2018 |
| On-Here Limited | Information & communication | 2019 |
| On-Line On-Track Limited | Extraterritorial | 2025 |
| Once Upon a Time (Staffordshire) Limited | Health & social care | 2023 |
| Once Upon a Time Photography Limited | Professional & scientific | 2019 |
| One Agency Estate Agents Limited | Real estate | 2018 |
| One Bill Property Management Services Ltd | Real estate | 2026 |
| One Contracts Ltd | Manufacturing | 2017 |
| One Delivery Staffordshire Limited | Wholesale & retail | 2018 |
| One Electrical Engineering Ltd | Manufacturing | 2018 |
| One Escape Ltd | Arts & recreation | 2024 |
| One Life Investments Limited | Real estate | 2018 |
| One London (GB) Ltd | Real estate | 2013 |
| One Stop Marketing (Stoke) Ltd | Professional & scientific | 2025 |
| One Stop Nutrition Ltd | Wholesale & retail | 2025 |
| One Sure Group Limited | Financial services | 2025 |
| One Universal Group Limited | Information & communication | 2026 |
| One W Investments Limited | Real estate | 2025 |
| One Way Build Limited | Construction | 2018 |
| One Way Holdings Limited | Financial services | 2021 |
| One Wedding Limited | Information & communication | 2012 |
| One With Nature Spa & Massage Limited | Other services | 2022 |
| One Wong Limited | Wholesale & retail | 2023 |
| One World for ALL Limited | Health & social care | 2024 |
| Onebomb ABC CIC | Arts & recreation | 2023 |
| Oneweb Limited | Real estate | 2009 |
| Online Environmental Limited | Water & waste | 2025 |
| Online Flooring UK Ltd | Construction | 2022 |
| Online Presence Agency Limited | Information & communication | 2025 |
| Only 4 Humans Limited | Wholesale & retail | 2026 |
| Only Faces by JB Ltd | Other services | 2025 |
| Only Vans Media Services Ltd | Professional & scientific | 2025 |
| Only Vanz Ltd | Manufacturing | 2023 |
| Only Your Shop Limited | Wholesale & retail | 2023 |
| Ontherise Technologies Ltd | Information & communication | 2025 |
| Opal Experience Ltd | Wholesale & retail | 2026 |
| Opal Financials Commercial Solutions Ltd | Financial services | 2022 |
| Open Mind Care Company Ltd | Health & social care | 2023 |
| Open Space Concepts Ltd | Construction | 2018 |
| Openpetase CIC | Water & waste | 2024 |
| Opera Security Ltd | Administrative & support | 2024 |
| OPH Health & Wellbeing Ltd | Other services | 2024 |
| Opti Engineering Limited | Professional & scientific | 2015 |
| Opticalprogrammer.Com Ltd | Manufacturing | 2021 |
| Optim Transport Ltd | Transport & logistics | 2022 |