| Nuvolo Group Ltd | Information & communication | 2025 |
| NWR Electrical Limited | Construction | 2014 |
| NY Sales Limited | Professional & scientific | 2010 |
| Nykang Ltd | Real estate | 2025 |
| O'Dwyer Builders Ltd | Construction | 2007 |
| O'Neill Ward Properties Ltd | Real estate | 2024 |
| O.P Homes Ltd | Real estate | 2021 |
| OAD Roofing Limited | Construction | 2026 |
| OAF Flooring Limited | Construction | 2023 |
| OAKBRAE10 Limited | Administrative & support | 2024 |
| Oakes Leisure Ltd | Professional & scientific | 2008 |
| Oakham Brickwork Limited | Construction | 1999 |
| Oakham Developments (West Midlands) Ltd | Extraterritorial | 2016 |
| Oakham Grange Management Company Limited | Hospitality | 1992 |
| Oakham Investments Holdings Limited | Financial services | 2020 |
| Oaklands (Wombourne) Management Company Limited | Households | 2010 |
| Oaklawn Guest House Limited | Hospitality | 2011 |
| Oakley's Farm Shop Ltd | Wholesale & retail | 2025 |
| Oakridge Holdings Limited | Real estate | 2024 |
| Oaktree Technology Ltd | Administrative & support | 2008 |
| Oakwell Estates Ltd | Construction | 2014 |
| Oakwell Workforce Ltd | Administrative & support | 2026 |
| OAX Electrical Systems Limited | Administrative & support | 2021 |
| Occupational Hygiene Services Limited | Professional & scientific | 2022 |
| Ocean Display Limited | Administrative & support | 2011 |
| Ocean Interiors (Solihull) Ltd | Construction | 2013 |
| Ocean Kitchens Ltd | Construction | 2013 |
| Odegon Technologies Limited | Manufacturing | 2005 |
| Odyssey Investments Ltd | Financial services | 2024 |
| Ohic Limited | Professional & scientific | 2019 |
| OJA Consultancy Services Limited | Information & communication | 2008 |
| Okkoco Limited | Extraterritorial | 2016 |
| Old Basing Mews Limited | Real estate | 2018 |
| Old Northwick Stables Limited | Agriculture & fishing | 2009 |
| Old Quarter Property Renovations & Maintenance Ltd | Construction | 2025 |
| Oldswinford Electrical Services Limited | Construction | 2013 |
| Olipen Holdings Limited | Financial services | 2025 |
| Olipen Investments Ltd | Real estate | 2022 |
| Olipen Ltd | Real estate | 2019 |
| Oliver & Carter Limited | Financial services | 2015 |
| Oliver Johnson Homes Limited | Real estate | 2016 |
| Oliver Richard Properties Limited | Real estate | 2026 |
| OLO Solutions Limited | Administrative & support | 2021 |
| OLS IT Services Ltd | Wholesale & retail | 2013 |
| Omega Electrical Services Limited | Construction | 2016 |
| Omega Properties (Midlands) Ltd | Real estate | 2024 |
| Omni HR Limited | Professional & scientific | 2018 |
| Omnisity DMS Limited | Education | 2011 |
| Omnisity Limited | Information & communication | 2002 |
| One Big Idea Ltd | Professional & scientific | 2009 |
| One Carpart Limited | Wholesale & retail | 2017 |
| One Pack Custom Designs Ltd | Manufacturing | 2024 |
| One Shot Charlies Limited | Other services | 2015 |
| Ontrac Digital Limited | Manufacturing | 2006 |
| Open Across Home Trading Ltd | Wholesale & retail | 2021 |
| Opened Clothing Ltd | Wholesale & retail | 2025 |
| Opening Doors & Venues Limited | Administrative & support | 2019 |
| Operations Management Services Limited | Professional & scientific | 2024 |
| Optimal Paraplanning Ltd | Financial services | 2025 |
| Optimise Connections Ltd | Wholesale & retail | 2023 |
| Optimum Consultancy and Education Ltd | Administrative & support | 2012 |
| Orchard Financial Advisers Limited | Financial services | 2014 |
| Orchard Legal Partnership LLP | | 2024 |
| Orchard Wealth Management Limited | Financial services | 2026 |
| Orchid Financial Solutions Ltd | Professional & scientific | 2007 |
| Oriel Healthcare Limited | Health & social care | 2017 |
| Orion Cybersecurity Ltd | Information & communication | 2023 |
| Orphic Brand Ltd | Wholesale & retail | 2024 |
| Orthum Limited | Health & social care | 2007 |
| Osei Minkah Housing Limited | Real estate | 2019 |
| OSU Trans Ltd | Transport & logistics | 2026 |
| Otis Civils Limited | Professional & scientific | 2025 |
| Otis Environmental Limited | Extraterritorial | 2011 |
| Otis Events Ltd | Extraterritorial | 2023 |
| Otis Exhibitions Ltd | Extraterritorial | 2023 |
| Otis Group Holdings Limited | Administrative & support | 2017 |
| Otis Group Services Limited | Manufacturing | 1999 |
| Otis Media Ltd | Information & communication | 2021 |
| Otis Tanks Ltd | Administrative & support | 2020 |
| Ottenbridge Ltd | Professional & scientific | 2022 |
| Otus Interior Design Limited | Professional & scientific | 2021 |
| OUR Business Services Ltd | Administrative & support | 2022 |
| OUR Edge Education Limited | Education | 2022 |
| OUR Footsteps Ltd | Health & social care | 2023 |
| OUR Heritage Orchard CIC | Health & social care | 2026 |
| OUR Lifestyle Magazine Limited | Information & communication | 2017 |
| OUR Plaice Limited | Hospitality | 2008 |
| OUR Village Green Ltd | Wholesale & retail | 2021 |
| OUT Class Barber Ltd | Other services | 2022 |
| OUT in the Field Ltd | Professional & scientific | 2022 |
| OUT of the Blue Fabrications Ltd | Construction | 2021 |
| Outcomes (U.K.) Limited | Professional & scientific | 2012 |
| Outdoor Advertising Media Ltd | Professional & scientific | 2014 |
| Outpost Motorcycles Ltd | Wholesale & retail | 2009 |
| Outpost Motorcycles West Midlands Ltd | Wholesale & retail | 2025 |
| Outsource Your FD Limited | Professional & scientific | 2024 |
| Oval Maintenance Services Limited | Other services | 2001 |
| Overarch Developments Ltd | Construction | 2016 |
| Overarch Property Limited | Real estate | 2016 |
| Ovin Limited | Administrative & support | 2022 |