| Nutravolve Ltd | Extraterritorial | 2025 |
| Nutriessence Ltd | Health & social care | 2025 |
| Nutrifit UK Ltd | Health & social care | 2025 |
| NV Property Limited | Real estate | 2018 |
| NXT GEN Coaching Ltd | Arts & recreation | 2025 |
| NYX Clothing London Ltd | Wholesale & retail | 2025 |
| O J Metals Limited | Wholesale & retail | 2012 |
| O&M Fire Protection Ltd | Construction | 2024 |
| O'Brien New Homes Limited | Construction | 2023 |
| O. D. Barbers Ltd | Other services | 2026 |
| O.E.S. (London) Limited | Wholesale & retail | 1961 |
| Oak & Stone - the Landscape Company Limited | Administrative & support | 2021 |
| Oak Facility Engineers Ltd | Construction | 2026 |
| Oak Galleries Ltd | Real estate | 2020 |
| Oakdene Contract Services Limited | Administrative & support | 2015 |
| Oakdin (UK) Limited | Health & social care | 2002 |
| Oakdin Montessori Kindergarten Limited | Education | 2002 |
| Oakdin of London Importers & Exporters Co. Ltd. | Wholesale & retail | 2001 |
| Oakfields Partners Ltd | Administrative & support | 2019 |
| Oakhurst Residents Management Company Limited | Households | 2022 |
| Oaklands Court (Welwyn) Freehold Limited | Hospitality | 2015 |
| Oaklands Hamlet Resident Management Limited | Households | 2016 |
| Oaklands Park Engineering Limited | Professional & scientific | 2022 |
| Oakleigh Joinery Essex Limited | Manufacturing | 2025 |
| Oaksure Security Limited | Administrative & support | 2017 |
| Oakwood Heights Residents Company Limited | Households | 1998 |
| Oakwood Intelligence Ltd | Other services | 2011 |
| OBE Resource Ltd | Professional & scientific | 2026 |
| OBG Access Ltd | Manufacturing | 2023 |
| Obiesili Foods Ltd | Hospitality | 2023 |
| Obross Ltd | Hospitality | 2025 |
| Obs-One Ltd | Manufacturing | 2018 |
| Obscura Theatre Ltd | Administrative & support | 2022 |
| Obsolve Ltd | Manufacturing | 2022 |
| OC Vision Ltd | Information & communication | 2025 |
| Occasio Consulting Ltd | Information & communication | 2024 |
| Occupational Therapy Essex Ltd | Health & social care | 2026 |
| Ocean Bell Consultants Limited | Professional & scientific | 2023 |
| Oceancroft Limited | Construction | 2017 |
| Ochre Salon Ltd | Other services | 2022 |
| OCK Properties Ltd | Real estate | 2026 |
| Ocrab Limited | Other services | 2015 |
| Octio Limited | Administrative & support | 2009 |
| ODD Muse Limited | Wholesale & retail | 2020 |
| Odinium Limited | Information & communication | 2000 |
| Odus (UK) Limited | Wholesale & retail | 2004 |
| Odyssey Learning and Development Limited | Professional & scientific | 2013 |
| OED Consulting Limited | Professional & scientific | 1998 |
| OF the Girls Ltd | Wholesale & retail | 2019 |
| OFF Licensed FM Ltd | Information & communication | 2025 |
| Off-Lineldn Ltd | Wholesale & retail | 2025 |
| Ofoma Ltd | Health & social care | 2010 |
| Ofrise Limited | Hospitality | 2017 |
| OGL Ground Engineering Ltd | Construction | 2019 |
| Ogundehin Consults Ltd | Professional & scientific | 2020 |
| OH2 Services Ltd | Administrative & support | 2018 |
| Ohara Davies Management Ltd | Professional & scientific | 2017 |
| Oilfield Solutions (UK) Limited | Professional & scientific | 2010 |
| OJ Building Services Limited | Construction | 2022 |
| OJ Consulting Limited | Extraterritorial | 2013 |
| OJ Fire Doors Ltd | Construction | 2026 |
| OJ Firestopping Ltd | Construction | 2024 |
| OJD Consultants Limited | Information & communication | 2007 |
| Okati Medical Services Limited | Health & social care | 2009 |
| Olaide Margaret Ltd | Financial services | 2018 |
| Old Brentwoods Club | Education | 2025 |
| Old Brentwoods Club Trading Limited | Hospitality | 2026 |
| Old Brentwoods Society | Real estate | 2025 |
| Old Macdonalds Farm & FUN Park Limited | Arts & recreation | 2016 |
| Oldfields Holdings Ltd | Real estate | 2021 |
| Oldwynn Limited | Arts & recreation | 2014 |
| Olehbra Properties Limited | Real estate | 2023 |
| Olga Express Ltd | Transport & logistics | 2018 |
| Olicav Ltd | Information & communication | 2020 |
| Olig Ltd | Administrative & support | 2017 |
| Olihew Properties Ltd | Real estate | 2024 |
| Olimar Management Company Limited | Households | 2024 |
| Olivaw Finance Holdings Limited | Financial services | 2023 |
| Olivaw Finance Ltd | Financial services | 2019 |
| Olive Hermanos Limited | Wholesale & retail | 2025 |
| Oltre Limited | Health & social care | 2025 |
| OM Beers & Minerals Ltd | Wholesale & retail | 2003 |
| OM LOG (UK) Ltd | Transport & logistics | 2023 |
| Omar Malik Ltd | Other services | 2025 |
| OME Management Ltd | Professional & scientific | 2024 |
| Omega Taxis Ltd | Transport & logistics | 2010 |
| Omneality Limited | Information & communication | 2008 |
| Omni Holdings Limited | Construction | 1957 |
| Omnia Ubique Ltd | Professional & scientific | 2025 |
| ON the Limit Performance Ltd | Professional & scientific | 2023 |
| On-Line Ltd | Information & communication | 2017 |
| Onamds Care Limited | Health & social care | 2023 |
| One Avenue Acquisitions Limited | Professional & scientific | 2011 |
| One Avenue CL NR Limited | Real estate | 2024 |
| One Avenue Facilities Management Limited | Real estate | 2019 |
| One Avenue Finsbury Limited | Real estate | 2016 |
| One Avenue Greycoat Limited | Real estate | 2024 |
| One Avenue Holdings Limited | Professional & scientific | 2021 |
| One Avenue IT Services Limited | Real estate | 2015 |
| One Avenue Jewry Limited | Real estate | 2015 |