| Opus Management Company Limited | Households | 2002 |
| Oralgenics Limited | Health & social care | 2007 |
| Orange AL Ltd | Health & social care | 2026 |
| Orange Cactus Consulting Limited | Professional & scientific | 2017 |
| Orange Restaurant Ltd | Hospitality | 2026 |
| Orange Skies Ltd | Information & communication | 2012 |
| Orange Square Estates Limited | Real estate | 2010 |
| Orbic Bearings Limited | Wholesale & retail | 1985 |
| Orbit Electronics Limited | Wholesale & retail | 1990 |
| Orbit Food and Beverage Ltd | Wholesale & retail | 2022 |
| Orbs of Gold Consultancy Limited | Information & communication | 2015 |
| Orchard & Friends Limited | Wholesale & retail | 2024 |
| Orchard Gate (Dartford) Management Company Limited | Households | 1991 |
| Orchard Leaze Management Company Limited | Households | 2011 |
| Orchard Manor (Management Company) Limited | Households | 2022 |
| Orchard Ortho Property Limited | Real estate | 2019 |
| Orchard Orthodontics Limited | Health & social care | 2011 |
| Orchard Way (Shirley) (Courts C D H J P) Residents Association Limited | Households | 1968 |
| Orchard Way (Shirley) (Courts Q,R,S) Residents Association Limited | Hospitality | 1967 |
| Orchard Works Residents Company Limited | Households | 1987 |
| Orchestrating Identity Limited | Information & communication | 2022 |
| Orchid Fields Swindon Management Company Limited | Households | 2004 |
| Orchid MS | Real estate | 2013 |
| Orchidquest Limited | Households | 1996 |
| Oree Mart Ltd | Wholesale & retail | 2021 |
| Oree Tea Ltd | Hospitality | 2024 |
| Oreklev Ltd | Information & communication | 2018 |
| Orel Investments Ltd | Real estate | 2017 |
| Organic Beginnings Limited | Hospitality | 2014 |
| Organic Design & Digital Brand Agency Ltd | Arts & recreation | 2019 |
| Organicent Ltd | Administrative & support | 2024 |
| Organised GEM Planners Ltd | Wholesale & retail | 2022 |
| ORI Olokun Ltd | Health & social care | 2019 |
| Oriental Choice Limited | Professional & scientific | 2009 |
| Oriental Coffee and Tea Ltd | Hospitality | 2014 |
| Origbemi UK Ltd | Wholesale & retail | 2025 |
| Origin Health Care Group Limited | Health & social care | 2025 |
| Original Beauty & Hair Ltd | Other services | 2010 |
| Orion Precision (Basildon) Limited | Manufacturing | 2013 |
| Orion Sterling Ltd | Extraterritorial | 2024 |
| Orion Surveying Ltd | Professional & scientific | 2025 |
| Orionis Developments Limited | Real estate | 2020 |
| Oris Waste Removal Ltd | Water & waste | 2025 |
| Oriscia Holdings Ltd | Manufacturing | 2023 |
| Orisoun Minerals & Resources Limited | Mining & quarrying | 2018 |
| Orissa Capital Partners Limited | Extraterritorial | 2015 |
| Orlando Drive (Basildon) Residents Association Limited | Households | 1988 |
| Orlando Henry Enterprise Ltd | Professional & scientific | 2022 |
| Ormco Associated Investments Limited | | 1986 |
| Ornva & Co Ltd | Manufacturing | 2021 |
| Oroko Ltd | Financial services | 2012 |
| Orra Collections Limited | Other services | 2022 |
| Orsett Contracts Limited | Construction | 2017 |
| Ortex Management Limited | Real estate | 2024 |
| Orvanta Construction Ltd | Construction | 2026 |
| Osaka Transport Ltd | Transport & logistics | 2023 |
| Osala Limited | Professional & scientific | 2016 |
| Osborn Glass and Windows Limited | Manufacturing | 2009 |
| Osborn Limited | Real estate | 2010 |
| Osborn Real Estate (Braintree) LLP | | 2007 |
| Osborn Securities (Chester) Limited | Real estate | 2008 |
| Osborn Securities Limited | Real estate | 1997 |
| Osborne Court Management Company (ST Ives) Limited | Households | 2012 |
| Osborne Flooring Limited | Construction | 2013 |
| Osborne Grove (Burnley) Management Company Limited | Households | 2004 |
| Osbourne Upholstery Limited | Wholesale & retail | 2018 |
| OSC Accounts Limited | Administrative & support | 2017 |
| Oscam Consulting Limited | Information & communication | 2014 |
| Oscar Denni Grooming Ltd | Other services | 2023 |
| Oscar Denni Ltd | Other services | 2013 |
| OSF International Social Care Services Ltd | Education | 2021 |
| Oshea Aesthetics Ltd | Other services | 2024 |
| Osibo Logistics Ltd | Transport & logistics | 2018 |
| Osier House Management Company (Corney Reach) Limited | Households | 1995 |
| Oskal Limited | Real estate | 2024 |
| Osman Jalloh & Partners Ltd | Professional & scientific | 2022 |
| Osong Limited | Manufacturing | 2023 |
| OSP GRP Holdings Ltd | Professional & scientific | 2019 |
| OSP GRP Ltd | Administrative & support | 2014 |
| OSP Mena Limited | Professional & scientific | 2019 |
| OSP Recruitment Limited | Administrative & support | 2019 |
| OSP Services GRP Ltd | Administrative & support | 2014 |
| OSR Chestnut Limited | Real estate | 2024 |
| Ossies Jerk Chicken Limited | Hospitality | 2023 |
| Osteopathic Clinic (Croydon) Limited | Health & social care | 2014 |
| Osteopathic Development Limited | Education | 2000 |
| Osteria Londra Ltd | Hospitality | 2016 |
| Osterley Court Management Company Limited | Households | 2003 |
| Ostler Court Management Company Limited | Households | 2009 |
| Otamra Limited | Administrative & support | 2018 |
| Otarukay Healthcare Services Limited | Health & social care | 2013 |
| Oterans Electricals Ltd | Construction | 2025 |
| Ottaj Consultancy Limited | Other services | 2020 |
| Ottoman Kebab Ltd | Hospitality | 2019 |
| Otul Drivers Drivers Association Limited | Transport & logistics | 2019 |
| Oudology London Limited | Wholesale & retail | 2019 |
| OUR Father’S House Limited | Education | 2024 |
| OUR Local Online Limited | Information & communication | 2019 |
| OUR MAP Limited | Hospitality | 2021 |
| OUR Village, OUR Community CIC | Education | 2025 |