| Oral Ceramics Limited | Health & social care | 1984 |
| Oran NA Mara Harris Limited | Hospitality | 2025 |
| Orange and Orange Ltd | Wholesale & retail | 2022 |
| Orange Blossom Florist Ltd | Wholesale & retail | 2021 |
| Orange Blossom Flowers Ltd | Wholesale & retail | 2025 |
| Orange Investment Group Limited | Real estate | 2019 |
| Orange Shop Northampton Limited | Wholesale & retail | 2026 |
| Orange Systems Limited | Administrative & support | 2025 |
| Orange Teapot Limited | Information & communication | 2025 |
| Orange Valley Limited | Real estate | 2022 |
| Orangeries Limited | Extraterritorial | 2000 |
| ORB People Limited | Administrative & support | 2018 |
| Orbel Property Services Ltd | Households | 2025 |
| Orbic International Limited | Manufacturing | 1988 |
| Orbis Build Ltd | Construction | 2024 |
| Orbit & ORK Limited | Information & communication | 2026 |
| Orbit Rail Ltd | Information & communication | 2024 |
| Orbit Technologies Limited | Information & communication | 2010 |
| Orbit Ventures Ltd | Real estate | 2002 |
| Orbital Safety Solutions Limited | Professional & scientific | 2021 |
| Orbitonics Limited | Professional & scientific | 2013 |
| Orcadian Healthcare Limited | Health & social care | 2009 |
| Orcadian Media Ltd | Professional & scientific | 2024 |
| Orchard Block Management Services Limited | Real estate | 1993 |
| Orchard Care Solutions Limited | Health & social care | 2025 |
| Orchard Day Nurseries Ltd. | Education | 2014 |
| Orchard Day Nursery Limited | Education | 1999 |
| Orchard Flooring Ltd | Construction | 2011 |
| Orchard Home Cleaning Ltd | Administrative & support | 2013 |
| Orchard House - Northampton Ltd | Health & social care | 2019 |
| Orchard Metal Fabrications LLP | | 2023 |
| Orchid Analytics Limited | Information & communication | 2023 |
| Orchid Fields (Biddenham Vale) Management Company Limited | Households | 2013 |
| Orchid Jewelry London Ltd | Manufacturing | 2021 |
| Ordnance House RTM Company Ltd | Hospitality | 2015 |
| Orea Apartments Ltd | Real estate | 2019 |
| Oregano Pizzeria Ltd | Hospitality | 2020 |
| Oreini Limited | Professional & scientific | 2018 |
| Oren PHO Ltd | Hospitality | 2018 |
| Organic Planet Valley Ltd | Wholesale & retail | 2021 |
| Orgress Limited | Information & communication | 2026 |
| ORI Beauty Studio Ltd | Other services | 2025 |
| Orien Management Ltd | Real estate | 2019 |
| Oriental Market Northampton Ltd | Wholesale & retail | 2018 |
| Origin and Oak Ltd | Wholesale & retail | 2025 |
| Origin and Sources Limited | Wholesale & retail | 2024 |
| Origin Business Finance Ltd | Financial services | 2019 |
| Origin Property (Primrose Hill Leasehold) Limited | Real estate | 2025 |
| Origin Property (Primrose Hill) Limited | Real estate | 2018 |
| Origlass UK Limited | Manufacturing | 2016 |
| Oriole Glass Holdings Limited | Financial services | 2017 |
| Ormiston House Ltd | Construction | 2018 |
| Ormonde Ensemble CIC | Information & communication | 2024 |
| Ormston Partners Ltd | Administrative & support | 2024 |
| Oror Limited | Wholesale & retail | 2019 |
| Orrön Energy Development Limited | Professional & scientific | 2023 |
| Ortem Supplies Limited | Wholesale & retail | 2018 |
| Ortho Limited | Health & social care | 2005 |
| Ortura Labs Ltd | Information & communication | 2012 |
| Osae Express Limited | Transport & logistics | 2026 |
| Osborn Design Limited | Professional & scientific | 2009 |
| Osborne Commercial Consultants Limited | Professional & scientific | 2014 |
| Osborne Fabrication Ltd | Manufacturing | 2015 |
| Oscar Supplies Limited | Wholesale & retail | 2002 |
| Oscarville Limited | Other services | 2017 |
| Osei Services Ltd | Health & social care | 2018 |
| Oseni Engineering Ltd | Professional & scientific | 2021 |
| Osit Limited | Information & communication | 2017 |
| Osit Realty Limited | Real estate | 2021 |
| Oska Research Consultancy Ltd | Information & communication | 2015 |
| Osmond Catering Limited | Hospitality | 2011 |
| Osner Consulting Ltd | Health & social care | 2024 |
| Ospheric Ltd | Professional & scientific | 2024 |
| OSR Limited | Hospitality | 2020 |
| Osso Logistics Limited | Transport & logistics | 1993 |
| Osyth Close Management Limited | Real estate | 2016 |
| OT33 Occupational Therapy Services Ltd | Health & social care | 2022 |
| Otago Cromwell Limited | Construction | 2002 |
| Otago Healthcare Limited | Professional & scientific | 2007 |
| OTB Developments Ltd | Construction | 2026 |
| OTE Installations Limited | Construction | 2019 |
| Otelli Limited | Professional & scientific | 2014 |
| OTG Foods Limited | Hospitality | 2006 |
| Other View Ltd | Construction | 2015 |
| Otiigo Trading Limited | Professional & scientific | 2019 |
| Otis News Agent Limited | Wholesale & retail | 2024 |
| OTO Car Sale Ltd | Wholesale & retail | 2023 |
| OTR Freight Ltd | Wholesale & retail | 2014 |
| OTT Internet Ltd | Information & communication | 2010 |
| Otter Stone Group Ltd | Real estate | 2017 |
| Otters Leisure (Northants) Ltd | Administrative & support | 2018 |
| Ottilia Holdings Limited | Financial services | 2024 |
| Ottoman Kitchen Northampton Limited | Real estate | 2022 |
| OTU Group Ltd | Administrative & support | 2022 |
| Otugaga Global Concepts Limited | Transport & logistics | 2022 |
| Otugaga Global Staffing Ltd | Administrative & support | 2024 |
| Otwe Naturals Limited | Wholesale & retail | 2023 |
| Oundle Solar Limited | Construction | 2023 |
| Oundle Wealth Management Ltd | Financial services | 2012 |
| OUO Studio Ltd | Wholesale & retail | 2026 |