| Opallais Ltd | Health & social care | 2016 |
| Opaque Properties Limited | Construction | 2018 |
| Open Door Environmental Education CIC | Education | 2026 |
| Open Doors Rental Ltd | Real estate | 2018 |
| Openlx SP Ltd | Professional & scientific | 2011 |
| Openshaw Internet Pharm Ltd | Health & social care | 2023 |
| Opensubject Limited | Real estate | 1999 |
| Opentrace Ltd. | Information & communication | 2025 |
| Openway Solutions Limited | Financial services | 2015 |
| Operex Ltd | Professional & scientific | 2026 |
| Operosus Ltd | Information & communication | 2025 |
| Opex Pro Ltd | Wholesale & retail | 2024 |
| Ophd Association | Other services | 2022 |
| Ophthalmic Service Ltd | Health & social care | 2019 |
| OPI Builders Ltd | Construction | 2021 |
| Opinow ARC Limited | Wholesale & retail | 2025 |
| Oprea Property Limited | Construction | 2019 |
| Opreal Advisory Group Ltd | Real estate | 2026 |
| OPS Hire Ltd | Administrative & support | 2024 |
| Optek Design Ltd. | Arts & recreation | 1998 |
| Optics Direct Limited | Health & social care | 2014 |
| Opticus Ltd | Wholesale & retail | 2018 |
| Optika Danek Ltd. | Wholesale & retail | 2016 |
| Optima Building Solutions Ltd | Administrative & support | 2015 |
| Optima Pool Limited | Manufacturing | 2013 |
| Optimal Houses Ltd | Real estate | 2024 |
| Optimal Trade Ltd. | Administrative & support | 2007 |
| Optimised Power Controls Limited | Other services | 1982 |
| Optimum Developments Limited | Administrative & support | 2011 |
| Optimum Electrical (NW) Ltd | Construction | 2022 |
| Optimum Hire Limited | Administrative & support | 2019 |
| Optimum Invest Limited | Hospitality | 2016 |
| Optimum Management Ltd | Professional & scientific | 2018 |
| Optimum Plus Consulting Ltd. | Administrative & support | 2011 |
| Optimum Technical Solutions Limited | Manufacturing | 2014 |
| Optimus Build Limited | Construction | 2010 |
| Optimus Training Ltd | Education | 2025 |
| OPU1ENT Ltd | Professional & scientific | 2019 |
| Opulent Imperial Shine Ltd | Administrative & support | 2026 |
| Opulent Investment Group Ltd | Real estate | 2023 |
| Opulent Living and Developments Ltd | Real estate | 2026 |
| Opulent Upholstery Ltd | Manufacturing | 2019 |
| Opulent Ventures Ltd | Real estate | 2024 |
| ORA Studios Group Ltd | Arts & recreation | 2026 |
| Oracle Estate Agent Limited | Real estate | 2017 |
| Oracle Fieldwork Limited | Professional & scientific | 2012 |
| Oracle Sales & Lettings Ltd | Real estate | 2025 |
| Orakzai Ltd | Wholesale & retail | 2024 |
| Oralcare Limited | Health & social care | 2000 |
| Orange Psychology Limited | Education | 2016 |
| Orben (Disley) Ltd | Hospitality | 2026 |
| Orben (Norden) Ltd | Hospitality | 2023 |
| Orben Holdings Ltd | Real estate | 2024 |
| Orben Limited | Hospitality | 2017 |
| Orchard Optical Limited | Wholesale & retail | 2017 |
| Orchard Rock Development Limited | Information & communication | 2014 |
| Orchard Street West Management Limited | Households | 2023 |
| Orchard View Joinery and Construction Ltd | Construction | 2025 |
| Orchids Florist Ltd | Wholesale & retail | 2017 |
| Ordacrest Limited | Real estate | 1967 |
| ORE Logistics Ltd | Transport & logistics | 2024 |
| Oria Slimming Center Limited | Health & social care | 2022 |
| Origin Garden Rooms Ltd | Construction | 2023 |
| Original Smart Solutions Limited | Wholesale & retail | 2019 |
| Orion Essential Distribution Ltd | Information & communication | 2024 |
| Orkid Ideas Limited | Wholesale & retail | 2010 |
| Orlando Fried Chicken Limited | Hospitality | 2016 |
| Ormit Solutions Ltd. | Information & communication | 2011 |
| Ormsby J Limited | Wholesale & retail | 2017 |
| ORO Paints Limited | Manufacturing | 2025 |
| Orobosa Services Limited | Transport & logistics | 2017 |
| Oros Logistics Ltd | Transport & logistics | 2025 |
| Oros Pro Services Ltd | Wholesale & retail | 2021 |
| ORS Properties Limited | Real estate | 2026 |
| Ortha Properties Limited | Hospitality | 2026 |
| Osagiecar Sales Limited | Wholesale & retail | 2018 |
| OSB Management Ltd | Extraterritorial | 2025 |
| Osborne Apartment Ltd | Real estate | 2020 |
| Oscarp Limited | Information & communication | 2014 |
| Oscar’S Classico Ltd | Wholesale & retail | 2024 |
| Oscillation Records Ltd | Information & communication | 2024 |
| Osher Investments Ltd | Real estate | 2023 |
| Osher Pilates Academy Ltd | Arts & recreation | 2023 |
| Oslar IT Limited | Professional & scientific | 2016 |
| OSP Property Holdings Ltd | Real estate | 2021 |
| Ossmann Properties Ltd | Real estate | 2026 |
| OTC Learning Ltd | Education | 2026 |
| OTG Civils and Groundworks Ltd | Construction | 2024 |
| Otis Transport Ltd | Transport & logistics | 2023 |
| Otium U.K. Limited | Construction | 2017 |
| OTR Service and Mots Ltd | Wholesale & retail | 2019 |
| OUD Box Oldham Ltd | Wholesale & retail | 2025 |
| OUR Chippy and Kebab Ltd | Hospitality | 2023 |
| OUR Community Oldham CIC | Information & communication | 2025 |
| OUR Community Wardrobe - Oldham CIC | Health & social care | 2025 |
| OUR Own Field Ltd | Agriculture & fishing | 2023 |
| OUR Place - Uppermill Limited | Hospitality | 2025 |
| OUR Song Limited | Extraterritorial | 2007 |
| OUR Stationers Limited | Professional & scientific | 2004 |
| Outcalyx PVT Ltd | Professional & scientific | 2025 |