| OBW Perera Limited | Professional & scientific | 2010 |
| Obyisa Ltd | Health & social care | 2019 |
| Occucom Limited | Information & communication | 1996 |
| Occumi Ltd | Information & communication | 2017 |
| Ocean Consulting PVT Ltd | Other services | 2018 |
| Ocean Health Solutions Limited | Professional & scientific | 2019 |
| Ocean Rgcc Ltd | Hospitality | 2026 |
| Ocean View (Walton-On-The-Naze) Ltd | Households | 1999 |
| Oceancaster Limited | Information & communication | 2024 |
| OCS. Scaffolding and Property Maintenance Ltd. | Real estate | 2025 |
| Octacars Ltd | Transport & logistics | 2023 |
| Octago Global Limited | Professional & scientific | 2025 |
| Octavia Gardens (Gazeley) Management Company Limited | Households | 2017 |
| Octink Group Limited | Extraterritorial | 2008 |
| Octink Limited | Extraterritorial | 2021 |
| Octopus Cabling Ltd. | Information & communication | 2002 |
| Octopus Firestop Ltd | Construction | 2025 |
| Oculex Group Ltd | Professional & scientific | 2025 |
| Oculus Advertising Limited | Professional & scientific | 2016 |
| Oculus Wealth Management (North Dorset) Limited | Financial services | 2020 |
| Odatone Music Limited | Information & communication | 2026 |
| ODD Bunch Media Limited | Professional & scientific | 2010 |
| ODD Creations Ltd | Information & communication | 2022 |
| ODL Ortho Limited | Administrative & support | 2023 |
| ODO Luxe Beauty & Aesthetics Ltd | Other services | 2025 |
| Odyssey 43 Management Ltd | Professional & scientific | 2016 |
| OEP Live Limited | Arts & recreation | 2020 |
| OFA Holdings Ltd | Financial services | 2018 |
| OFF Peak Limited | Extraterritorial | 2000 |
| OFF the Wall Coverings Limited | Manufacturing | 2010 |
| Offbeat Vending Ltd | Wholesale & retail | 2024 |
| Offcut Group Ltd | Wholesale & retail | 2025 |
| Offensive Miniatures Limited | Manufacturing | 2016 |
| Offgrid Industries Ltd | Manufacturing | 2026 |
| Office Ally Limited | Information & communication | 1999 |
| Offlend Partners Ltd | Information & communication | 2025 |
| Ofina Ltd | Construction | 2026 |
| OFW Property Ltd | Real estate | 2022 |
| OG Tyre and Repair Ltd | Wholesale & retail | 2025 |
| OGG Rise Ltd | Professional & scientific | 2025 |
| Ogmtrading Ltd | Wholesale & retail | 2020 |
| OH Motor Sport Ltd | Arts & recreation | 2017 |
| OH NO HE Isn'T Ltd | Arts & recreation | 2026 |
| OH Property Limited | Real estate | 2022 |
| Ohana Delta Void Specialists Ltd | Construction | 2023 |
| Ohmypawsuk Ltd | Other services | 2023 |
| OI! Licensing Ltd | Other services | 2018 |
| OIL & GAS Consultancy Service Limited | Professional & scientific | 1992 |
| Oinos Limited | Financial services | 2024 |
| OJ and RA Holdings Ltd | Financial services | 2023 |
| OJ Surveying Limited | Professional & scientific | 2018 |
| OJF Enterprises UK Ltd | Wholesale & retail | 2024 |
| OJL Music Ltd | Information & communication | 2019 |
| OJS Consultants Limited | Administrative & support | 2013 |
| OK Only Essentials Ltd | Wholesale & retail | 2022 |
| OK World Wide Ltd | Hospitality | 2019 |
| Okaro Consultancy Ltd | Professional & scientific | 2021 |
| Okes Innovation Limited | Information & communication | 2025 |
| Oknowz Ventures Ltd | Real estate | 2025 |
| Okuoba Tech Limited | Construction | 2020 |
| Olae Ltd | Real estate | 2017 |
| Olayemi Awoyinfa Care Limited | Health & social care | 2021 |
| Old Essex Barn Limited | Hospitality | 2020 |
| Old House Info Ltd | Administrative & support | 2002 |
| Old Magistrates Court Witham Management Company Limited | Real estate | 2015 |
| Old Station Road (Halesworth) Management Limited | Households | 2021 |
| Old Town Barbers Colchester Ltd | Other services | 2024 |
| Old Vineyard Limited | Manufacturing | 2012 |
| Old Wivenhoe Quay (NO.1) Management Company Limited | Households | 1998 |
| Old Wivenhoe Quay (NO.3) Management Company Limited | Households | 2000 |
| Old Wivenhoe Quay (NO.4) Management Company Limited | Households | 2000 |
| Old Wivenhoe Quay Estate Management Company Limited | Households | 1999 |
| OLE Engineering Ltd | Professional & scientific | 2019 |
| Oledo R&D Ltd | Professional & scientific | 2021 |
| Oliemm Serv Ltd | Construction | 2020 |
| Olisolda Ltd | Professional & scientific | 2025 |
| Oliva Construction Limited | Construction | 2019 |
| Oliva Contracts Limited | Construction | 2019 |
| Oliva Holdings Limited | Financial services | 2022 |
| Oliva Homes (Kingsland) Limited | Construction | 2024 |
| Oliva Homes Limited | Construction | 2019 |
| Oliva Property Limited | Real estate | 2021 |
| Oliva Rossa Limited | Wholesale & retail | 2024 |
| Olive House Interiors Limited | Wholesale & retail | 2025 |
| Oliver Brighten Ltd | Construction | 2023 |
| Oliver Burn Holdings Limited | Real estate | 2015 |
| Oliver Matthew Construction Ltd | Construction | 2023 |
| Oliver Roofing Services Limited | Construction | 2021 |
| Olivers Group Limited | Extraterritorial | 2024 |
| Olivers Grove (Stanway) Management Company Limited | Real estate | 2015 |
| Olivet Limited | Construction | 2003 |
| Olley Roofing Ltd | Construction | 2021 |
| Ollivant Consulting Ltd | Professional & scientific | 2025 |
| OLM Holdings Limited | Financial services | 2023 |
| Oluyinka Olutunde Ltd | Health & social care | 2024 |
| OM Financial Ltd | Financial services | 2018 |
| Omacareservices Limited | Health & social care | 2023 |
| Omega Child Services (OCS) Limited | Administrative & support | 2017 |
| Omega Finance Services Limited | Professional & scientific | 2025 |
| OMG HOW Cheap Ltd | Wholesale & retail | 2011 |