| Oasis Event Management Limited | Hospitality | 2013 |
| Oasis Vantage Properties Ltd | Real estate | 2025 |
| Oates Commercial Services Ltd | Administrative & support | 2026 |
| OAZ Aspiration Care Ltd | Health & social care | 2026 |
| OB Electrical Services Limited | Construction | 2016 |
| OB Trading International Ltd | Wholesale & retail | 2025 |
| OBADIAH117 Ministry C.I.C. | Other services | 2025 |
| Obasij Healthcare Services Ltd | Health & social care | 2019 |
| Obeng Solutions Ltd | Transport & logistics | 2026 |
| OBG Enterprises Limited | Wholesale & retail | 2024 |
| Objective: Protection Ltd | Wholesale & retail | 2025 |
| Obmma Ltd | Financial services | 2023 |
| Occupational Health Assessment Ltd | Health & social care | 2018 |
| Occuro Ltd | Administrative & support | 2026 |
| Ocean Bay Trading Ltd | Extraterritorial | 2023 |
| Ocean END Holdings Unlimited | Administrative & support | 2020 |
| Ocean Fish Restaurant Ltd | Hospitality | 2025 |
| Ocean Glow Nutrition Ltd | Extraterritorial | 2024 |
| Ocean Mist Care Limited | Professional & scientific | 2024 |
| Ocean Mist Dental Limited | Health & social care | 2024 |
| Ocean Trauma Counselling Limited | Health & social care | 2024 |
| Ocean Volt Services Ltd | Mining & quarrying | 2025 |
| Oceanbites Pioneer PTY Ltd | Professional & scientific | 2024 |
| Oceanic Saunas Limited | Wholesale & retail | 2004 |
| Oceanorganiser Limited | Transport & logistics | 1998 |
| Oceanslides Ltd | Administrative & support | 2019 |
| Oceantrek Ltd | Administrative & support | 2021 |
| Ocellaris Invest Ltd | Financial services | 2025 |
| Ochucare Ltd | Health & social care | 2023 |
| Ocker Holdings Limited | Health & social care | 2012 |
| OCSER25 Limited | Households | 2025 |
| Octa-Core Solutions Ltd | Manufacturing | 2026 |
| Octa-Ovidiu Trans Limited | Transport & logistics | 2014 |
| Octi 4 Limited | Real estate | 2022 |
| Octizens Ltd | Real estate | 2017 |
| Octopus Stage & SET Limited | Manufacturing | 2010 |
| OD Penn Limited | Hospitality | 2024 |
| Odheusa Ltd | Health & social care | 2018 |
| Odroid Ltd | Information & communication | 2011 |
| Off-Script Safeguarding CIC | Education | 2026 |
| Offey Limited | Real estate | 2020 |
| Office 28 Ltd | Real estate | 2015 |
| Offinies Ltd | Transport & logistics | 2019 |
| Offnet UK Managment Ltd | Real estate | 2024 |
| Ofueje Ltd | Transport & logistics | 2024 |
| OG 168 Ltd | Hospitality | 2025 |
| Ogixo Limited | Wholesale & retail | 2024 |
| Ograytic Limited | Financial services | 2022 |
| OH Mining Limited | Financial services | 2021 |
| Ohisdavina Ltd | Health & social care | 2020 |
| Ohmic Solar Power Limited | Energy | 2010 |
| OJ Construction Building Co. Ltd | Construction | 2024 |
| Oj&J Consultant Ltd | Professional & scientific | 2015 |
| Oj&J Properties Ltd | Real estate | 2019 |
| OJA Marketplace Ltd | Wholesale & retail | 2024 |
| OJA Property Management and Lettings Ltd | Real estate | 2024 |
| OJL Associates Limited | Professional & scientific | 2013 |
| OJL Technologies Limited | Information & communication | 2024 |
| OJP Solutions Limited | Administrative & support | 2017 |
| OJR Ventures Limited | Real estate | 2026 |
| Ojàà Ltd | Information & communication | 2026 |
| OK Fashion Ltd | Wholesale & retail | 2026 |
| OKL Limited | Construction | 2025 |
| Okobi ED Limited | Health & social care | 2024 |
| Okwy Auto Ltd | Wholesale & retail | 2025 |
| OL Properties Ltd | Real estate | 2021 |
| Olayaramos Ltd | Transport & logistics | 2019 |
| Old Coppice Ltd | Real estate | 2023 |
| Old Falling Management Limited | Administrative & support | 2025 |
| Old Hill Cafe Limited | Hospitality | 2025 |
| Old Manor Residents Association Limited (the) | Households | 1969 |
| Old Park Management Limited | Households | 2025 |
| Old School House Mews Management Company Limited | Real estate | 1996 |
| Old Wulfrunians Association Limited | Arts & recreation | 1967 |
| Old Wulfrunians Club Limited | Hospitality | 1988 |
| Old Wulfrunians Hockey Club Limited | Arts & recreation | 2008 |
| Old Wulfrunians Sport and Social Club Limited | Arts & recreation | 2026 |
| Old Wulfrunians Trading Ltd | Arts & recreation | 2026 |
| Oldacre Gardens (Newbold on Stour) Management Company Limited | Households | 2022 |
| Oldenstately Ltd | Wholesale & retail | 2021 |
| Oldham Property Investments Limited | Health & social care | 2010 |
| Olgarium Ltd | Administrative & support | 2024 |
| Oliedd Enterprises Ltd | Health & social care | 2023 |
| Olisjay & Ronniers Limited | Wholesale & retail | 2019 |
| Olive & Peach Properties Limited | Real estate | 2024 |
| Olive and Juice UK Ltd | Hospitality | 2026 |
| Olive and Ruby Limited | Wholesale & retail | 2022 |
| Olive DEN Properties Ltd | Hospitality | 2025 |
| Olive Fashion Boutique Ltd | Wholesale & retail | 2026 |
| Olivecross Healthcare Ltd | Health & social care | 2026 |
| Oliver Clarke Ltd | Construction | 2022 |
| Oliver Webb Limited | Manufacturing | 2022 |
| Oliversalesupport Ltd | Administrative & support | 2025 |
| Olton Mere Management Company Limited | Households | 1988 |
| OLU Oluwajobi Ltd | Health & social care | 2013 |
| Oluokun Limited | Public administration | 2022 |
| Oluwabusayomi Limited | Health & social care | 2024 |
| Olvantra International Ltd | Wholesale & retail | 2026 |
| Olympus Advisory Ltd | Information & communication | 2020 |
| Olympus Dairy UK Ltd | Wholesale & retail | 2016 |