| Obsidian Car Care Ltd | Manufacturing | 2022 |
| OC Industries Ltd. | Arts & recreation | 2013 |
| Occasion Makers Limited | Hospitality | 2023 |
| OCHRE7 Ltd | Construction | 2024 |
| Oci-Tech Global Ltd | Manufacturing | 2025 |
| Octane Media Ltd | Information & communication | 2026 |
| OD Carpentry Limited | Construction | 2014 |
| Oden Wilson Limited | Professional & scientific | 2010 |
| OFF the Tools Limited | Professional & scientific | 2020 |
| Off-Site Timber Frame Solutions Ltd | Construction | 2024 |
| OGM Recruitment Limited | Administrative & support | 2020 |
| OH SO Colourful Ltd | Manufacturing | 2024 |
| Ohana Mobile Cocktail Bar Limited | Hospitality | 2024 |
| Oideachas Ltd | Education | 2020 |
| Oikos ELM Investments Limited | Real estate | 2021 |
| OL Bookkeeping Ltd | Professional & scientific | 2020 |
| Olafam Auto's Ltd | Wholesale & retail | 2018 |
| Olafam Properties UK Ltd. | Real estate | 2017 |
| Old Dairy (Kimpton) Management Company Limited | Households | 2023 |
| Old Hunters Lodge Limited | Hospitality | 1994 |
| Oldham Limited | Hospitality | 2024 |
| Oliver Haulage Ltd | Transport & logistics | 2025 |
| Oliver Scott Interiors Ltd | Manufacturing | 2017 |
| Olivers Cleaning & Services Ltd | Administrative & support | 2025 |
| OLORIRODDY07 Ltd | Real estate | 2025 |
| Olss - Operator Licence Support Services Ltd | Professional & scientific | 2025 |
| Olympus Build Ltd | Construction | 2020 |
| Omega Importers Ltd | Wholesale & retail | 2017 |
| Omeka Properties Ltd | Construction | 2023 |
| Omnia Access Ltd | Professional & scientific | 2023 |
| Omnia Cleaning Ltd | Administrative & support | 2023 |
| Omnimesh Ltd | Information & communication | 2026 |
| Omnis Multiplus Services Ltd | Construction | 2024 |
| Omniscreens Ltd | Manufacturing | 2011 |
| Omonell Holdings Limited | Information & communication | 2022 |
| Onaj Care Limited | Health & social care | 2019 |
| Ondi Properties Ltd | Real estate | 2025 |
| One DRX Ltd | Wholesale & retail | 2024 |
| One Plus One Consulting Ltd | Professional & scientific | 2017 |
| One Shot Creative Limited | Professional & scientific | 2021 |
| One Trafford Ltd | Real estate | 2025 |
| ONE2ONE Tyres Limited | Wholesale & retail | 2024 |
| Onepoint Outsource Ltd | Administrative & support | 2024 |
| Onestop Machinery Group Ltd | Manufacturing | 2018 |
| Onestop Renewable Innovations Limited | Wholesale & retail | 2025 |
| Online Envelopes Limited | Wholesale & retail | 2004 |
| Only Professional Services Limited | Real estate | 2002 |
| Onside Management Limited | Arts & recreation | 1999 |
| Onsite Scaffold Services Ltd | Construction | 2025 |
| Ontoeducation Ltd | Education | 2026 |
| Onvit Nutrition Ltd | Wholesale & retail | 2023 |
| OO Holdings Limited | Real estate | 2020 |
| Opera Aylesbury Limited | Hospitality | 2023 |
| Operative Activity Kontrol Ltd | Other services | 2020 |
| Operators R US Ltd | Transport & logistics | 2016 |
| Opmco Ltd | Information & communication | 2022 |
| Opportunity Health Improvement Ltd | Extraterritorial | 2015 |
| Opris Auto Ltd | Construction | 2017 |
| Opsollo Ltd | Information & communication | 2023 |
| Optima Creative Limited | Professional & scientific | 2016 |
| Optimal Compliance Consultant Ltd | Professional & scientific | 2026 |
| Optimal Contracting Limited | Construction | 2025 |
| Optimal DRY Lining and Decorations Ltd | Construction | 2017 |
| Optimal New Homes Limited | Construction | 2021 |
| Optimal Pharma Ltd | Wholesale & retail | 2016 |
| Optimum X Limited | Information & communication | 2025 |
| Opulence Design Ltd | Construction | 2026 |
| Opulent Virtual Assistance Ltd | Real estate | 2022 |
| Orange Circle Creative Ltd | Professional & scientific | 2018 |
| Orbis Managed Solutions Limited | Administrative & support | 2018 |
| Orbis Medics Ltd | Other services | 2016 |
| ORC Russ Limited | Arts & recreation | 2020 |
| Orcdec Limited | Construction | 2014 |
| Orchard Garage Sandridge Limited | Wholesale & retail | 2009 |
| Orderwork Limited | Information & communication | 2005 |
| Oregon Partners Ltd | Professional & scientific | 2023 |
| Orion Business Finance Limited | Financial services | 2026 |
| Orion Peak Limited | Wholesale & retail | 2024 |
| ORK Limited | Households | 2009 |
| Orluna Holdings Limited | Manufacturing | 2022 |
| Orluna LED Technologies Limited | Manufacturing | 2017 |
| Osei Care Services Ltd | Health & social care | 2021 |
| Oseni and Sons Limited | Real estate | 2024 |
| Oskartrans Ltd | Transport & logistics | 2021 |
| Osprey Gate Management Company Limited | Households | 2016 |
| Osprey UK Associates Limited | Professional & scientific | 2019 |
| Osteo-Clinic (UK) Limited | Health & social care | 2006 |
| Osterley Service Centre Ltd | Manufacturing | 2026 |
| Osterley Tyres Ltd | Manufacturing | 2026 |
| Otterburn Traders Limited | Wholesale & retail | 2025 |
| Ouse Communications Ltd | Information & communication | 2023 |
| Ouzel Management Limited | Professional & scientific | 2014 |
| Overcoming OCD Limited | Health & social care | 2022 |
| OVI Windows Ltd | Construction | 2019 |
| Ovidiu Ginga Limited | Transport & logistics | 2018 |
| Oxford Prestige Limited | Wholesale & retail | 2019 |
| OZ Tiling Ltd | Construction | 2020 |
| Ozcetin Company Limited | Information & communication | 2019 |
| Ozzies Burger Ltd | Hospitality | 2020 |
| O’ Deux Saveurs Ltd | Hospitality | 2026 |