| Old Autos Limited | Wholesale & retail | 2008 |
| Old Bakery Photographic Studio Limited | Professional & scientific | 2018 |
| Old Catholic Apostolic Church | Professional & scientific | 2010 |
| Old Chapel Finance Limited | Professional & scientific | 2024 |
| Old Coach Square Limited | Households | 2000 |
| Old Friend Capital Limited | Real estate | 2017 |
| Old House Family Office Limited | Financial services | 2017 |
| Old Kiln Car Company Ltd | Wholesale & retail | 2015 |
| Old Rectory Mews Management Ltd | Extraterritorial | 2024 |
| Old School Itchingfield Limited | Construction | 2026 |
| Old Town Mews Management Company Limited | Real estate | 1988 |
| Oldwell Sandhurst Management Company Limited | Households | 2021 |
| Oliac Solutions Limited | Wholesale & retail | 2024 |
| Olienda Ltd | Wholesale & retail | 2025 |
| Olive Accounts & Advisory Ltd | Professional & scientific | 2024 |
| Olive Branch Consultancy Limited | Administrative & support | 2017 |
| Olive Skye Events Limited | Hospitality | 2024 |
| Oliver Daniel Limited | Real estate | 2008 |
| Olizzy Ltd | Information & communication | 2012 |
| Ollie Noscoe Arborist Ltd | Agriculture & fishing | 2022 |
| Olliver Construction & Conversions Ltd | Construction | 2019 |
| Olokor Technology Limited | Wholesale & retail | 2024 |
| Olotila Ltd. | Other services | 2014 |
| Oluwanifesimi Ltd. | Manufacturing | 2023 |
| Omama Noor Limited | Wholesale & retail | 2025 |
| Omaze BR Limited | Wholesale & retail | 2025 |
| Ombom Ltd | Manufacturing | 2023 |
| OMC Automotive Limited | Wholesale & retail | 2023 |
| Omega Centre (Wareham) Management Company Limited | Real estate | 2023 |
| Omfax Limited | Information & communication | 1987 |
| Omfax Systems Limited | Information & communication | 1987 |
| Ommani Autism C.I.C. | Education | 2019 |
| Omni One Limited | Health & social care | 2022 |
| Omniaqo Ltd | Information & communication | 2018 |
| Omnico Creative Ltd | Information & communication | 2023 |
| Omnicorp Holdings Ltd | Real estate | 2021 |
| Omnika BR Ltd | Wholesale & retail | 2025 |
| Omnipilates Ltd | Other services | 2025 |
| Omnium Urology LLP | | 2021 |
| Omshree Ganesh Ltd | Hospitality | 2019 |
| OMV Enterprises Ltd | Health & social care | 2025 |
| Omvita Labs Ltd | Wholesale & retail | 2017 |
| ON the Box PAY Ltd | Administrative & support | 2021 |
| ON the Move (Exchange) Limited | Real estate | 2015 |
| One Call Cleaners Limited | Administrative & support | 2026 |
| One Choice Healthcare Ltd | Administrative & support | 2024 |
| One Four the Road - Sandbanks Limited | Arts & recreation | 2014 |
| One Gateway Ltd | Information & communication | 2012 |
| One GK Limited | Arts & recreation | 2026 |
| One Labs Ltd | Information & communication | 2020 |
| One Marine Limited | Wholesale & retail | 2023 |
| One Planet Herbs Ltd | Wholesale & retail | 2024 |
| One Rebellion Ltd | Financial services | 2020 |
| One River Ltd | Wholesale & retail | 2013 |
| One Step Further Creative Ltd | Professional & scientific | 2024 |
| One to One Food Services Limited | Wholesale & retail | 2006 |
| Onebox Ventures LLP | | 2020 |
| Onehouse Creative Ltd | Professional & scientific | 2018 |
| Onemove Property Ltd | Real estate | 2021 |
| Ones Upon a Motion Ltd | Manufacturing | 2025 |
| ONESIX16 Ltd | Wholesale & retail | 2023 |
| Onetec Limited | Administrative & support | 2010 |
| Ongas Guildford Ltd. | Other services | 2014 |
| Onion Tech Limited | Information & communication | 2026 |
| Online Guides UK Ltd | Information & communication | 2025 |
| Online Lifestyle Guides Ltd | Financial services | 2025 |
| Online Shop Ltd | Wholesale & retail | 2024 |
| Only Smile Locum Ltd | Administrative & support | 2024 |
| Only UP Limited | Professional & scientific | 2018 |
| Onreserve Limited | Administrative & support | 2026 |
| Onyx Bars Limited | Hospitality | 2019 |
| Onyx Events Limited | Professional & scientific | 2020 |
| Onyx G Limited | Information & communication | 2016 |
| Onyx Marine Limited | Administrative & support | 1995 |
| Onyx Marine Lymington Limited | Administrative & support | 2020 |
| Onyx Veterinary Ltd | Professional & scientific | 2025 |
| Onyxsis Limited | Information & communication | 2014 |
| Oodlemadoodles Creations Limited | Wholesale & retail | 2021 |
| Oodles Ltd | Wholesale & retail | 2020 |
| Oodo Ltd | Information & communication | 2021 |
| Ooops! Automotive Ltd | Wholesale & retail | 2022 |
| Ooops.Net Limited | Professional & scientific | 2011 |
| OP Customs Ltd | Manufacturing | 2025 |
| OP Ecom Limited | Wholesale & retail | 2020 |
| Oparade Professional Services Limited | Professional & scientific | 2021 |
| OPB Studio Ltd | Wholesale & retail | 2025 |
| Opees Group Ltd | Professional & scientific | 2021 |
| Open Digital Consulting Co Ltd | Information & communication | 2012 |
| Open Sea Property Group Ltd | Real estate | 2022 |
| Open Source Digital Scala Software Engineering Ltd | Professional & scientific | 2017 |
| Openbacklog Ltd | Information & communication | 2025 |
| Openshaw Property Ltd | Real estate | 2022 |
| Ophi 1 Ltd | Professional & scientific | 2026 |
| Ophir Finance Company Limited | Construction | 1958 |
| Ophir Property Limited | Real estate | 2015 |
| OPR Transport Limited | Transport & logistics | 2025 |
| Opses Limited | Professional & scientific | 2015 |
| Optech-Holdings Ltd | Financial services | 2024 |
| Opticus Optometry Services Ltd | Health & social care | 2011 |
| Optima Medical Aesthetics Ltd | Health & social care | 2021 |