| Occulta Limited | Wholesale & retail | 2026 |
| Ocean Capital Exchange Ltd | Administrative & support | 2018 |
| Ocean Financial Solutions Ltd | Administrative & support | 2017 |
| Oceanlink Trading Ltd | Wholesale & retail | 2023 |
| Ocius Solutions Ltd | Professional & scientific | 2019 |
| Ociyo Limited | Information & communication | 2023 |
| OCM Housing Ltd | Real estate | 2006 |
| OCO Connect CIC | Education | 2024 |
| OCO Limited | Construction | 1979 |
| OCR Global Property Services Ltd | Real estate | 2017 |
| OCR Management Consultants Limited | Professional & scientific | 2017 |
| Ocsystems Ltd | Information & communication | 2025 |
| Octavian Group Ltd | Real estate | 2018 |
| Octo Financial Planning Ltd | Financial services | 2022 |
| Octopus Developers Ltd | Construction | 2025 |
| OCTOPUS84 Ltd | Other services | 2016 |
| ODA Grill House Ltd | Hospitality | 2026 |
| Odapt Limited | Professional & scientific | 2025 |
| Odeis Limited | Information & communication | 2024 |
| Odera Services Limited | Real estate | 2021 |
| ODI Logistics Limited | Transport & logistics | 2025 |
| Odyssea A.B. Ltd | Construction | 2025 |
| OFF Group Limited | Other services | 2010 |
| OFF M2 Ltd | Real estate | 2025 |
| Office Masters Binding Supplies Limited | Wholesale & retail | 2013 |
| Office Oracle Group Limited | Administrative & support | 2011 |
| Office Space Graphics Ltd | Manufacturing | 2019 |
| Offo Services Ltd | Health & social care | 2022 |
| OGJ Machinery Pro Ltd | Wholesale & retail | 2025 |
| OGU Carpentry Limited | Construction | 2024 |
| Ohana Recruitment Limited | Administrative & support | 2021 |
| OHM Group Limited | Wholesale & retail | 2025 |
| OHZ Property Ltd | Real estate | 2023 |
| OJ EYE Ltd | Education | 2025 |
| OK Abims Ltd | Information & communication | 2025 |
| Okey- Philip Properties Limited | Construction | 2025 |
| Okomela IT Limited | Information & communication | 2008 |
| OLA Prestige Care Services Ltd | Health & social care | 2024 |
| Olabisi Limited | Professional & scientific | 2024 |
| Olaiya Foods and Catering Services Limited | Hospitality | 2025 |
| Olamat Ltd | Real estate | 2020 |
| Olamz Consulting Limited | Health & social care | 2021 |
| Olani Clinic Ltd | Health & social care | 2025 |
| Old Forge Garage (Belverdere) Ltd | Wholesale & retail | 2023 |
| Old Parsonage Mews Residents Management Limited | Households | 2020 |
| Oldcorn Educational Associates Limited | Education | 2012 |
| Olivias Hair Salon Ltd | Other services | 2017 |
| Oloton Property Limited | Real estate | 2020 |
| Oluglo Limited | Other services | 2015 |
| Olusimple Ltd | Manufacturing | 2011 |
| Olutoyek Ltd | Health & social care | 2023 |
| Olympia Leisure Limited | Arts & recreation | 1999 |
| Olympia Tools (UK) Limited | Wholesale & retail | 2006 |
| Olympiad Signs Limited | Manufacturing | 1966 |
| Olympio FS Limited | Information & communication | 2022 |
| Olympus Playgroup Limited | Education | 2015 |
| OMA Rissa Services Ltd | Information & communication | 2021 |
| Oma's & Co Ltd | Health & social care | 2014 |
| Omarich Ltd | Health & social care | 2023 |
| Oma’S & Co Property Investments Ltd | Hospitality | 2025 |
| Omega Grounds Care Ltd | Administrative & support | 2025 |
| Omega Merchant Ltd | Wholesale & retail | 2024 |
| Omega Tree Care Ltd | Administrative & support | 2022 |
| Omeru Ltd | Information & communication | 2026 |
| Ominee Services Limited | Other services | 2013 |
| OMJ Consulting Ltd | Information & communication | 2013 |
| Omni Webworks Limited | Information & communication | 2024 |
| Omnistar Consulting Ltd | Professional & scientific | 2026 |
| Omo-Aje Ltd | Real estate | 2018 |
| Omonx Speech and Language Therapy Ltd | Health & social care | 2026 |
| Omotone Ltd | Real estate | 2022 |
| Ondemandhousing Ltd | Real estate | 2024 |
| One Analytics Limited | Information & communication | 2024 |
| One Electrical Installations Ltd | Construction | 2007 |
| One Electrical Test Inspection Ltd | Construction | 2019 |
| One Family Care Limited | Health & social care | 2025 |
| One Fire & Security Ltd | Administrative & support | 2022 |
| One Flame GAS Engineers Ltd | Construction | 2022 |
| One Force Consulting Limited | Information & communication | 2025 |
| One Mind at a Time Limited | Health & social care | 2025 |
| One Mood Music Ltd | Arts & recreation | 2025 |
| One Step Before the Next Ltd | Administrative & support | 2026 |
| One Touch Cleaning Ltd | Other services | 2017 |
| One Vitality Group Limited | Wholesale & retail | 2022 |
| Onehub Facilities Ltd | Administrative & support | 2026 |
| Online Reference Control Limited | Information & communication | 2021 |
| Online Teaching & Assessment Limited | Education | 2018 |
| Onload Electrical Limited | Construction | 2009 |
| Only PAY What IT Weighs Limited | Water & waste | 2023 |
| Only Way IS Glam Limited | Other services | 2017 |
| Onlyrough Limited | Wholesale & retail | 2024 |
| Onufri Ltd | Transport & logistics | 2024 |
| Onyx Ultrasonics Limited | Professional & scientific | 2025 |
| OO Group Holdings Ltd | Wholesale & retail | 2023 |
| Ooberstock Limited | Wholesale & retail | 2010 |
| OOM Properties Ltd | Real estate | 2021 |
| Opal Refurbishment Limited | Construction | 1997 |
| Opal Ridge Ltd | Real estate | 2025 |
| OPB Services Limited | Health & social care | 2021 |
| Opeg Limited | Administrative & support | 2023 |