| Novomatic Gaming UK Limited | Other services | 2018 |
| Novomatic UK Ltd | Financial services | 1993 |
| NOW YOU Creative Limited | Arts & recreation | 2021 |
| Npughconsulting Ltd | Construction | 2025 |
| Nroseconsulting Ltd | Information & communication | 2025 |
| NS Fuel & Shop Ltd | Wholesale & retail | 2020 |
| NT Props Ltd | Information & communication | 2003 |
| Ntec (Bristol) Limited | Information & communication | 2016 |
| Nuts About Cinnamon Ltd | Wholesale & retail | 2026 |
| Nuvision Energy (Wales) Ltd | Construction | 2017 |
| NVC Investments Ltd | Financial services | 2022 |
| NVC Sourcing Ltd | Real estate | 2023 |
| NXN Executive Holdings Limited | Professional & scientific | 2024 |
| Nyabango Limited | Health & social care | 2016 |
| O M H Financial Ltd | Financial services | 2025 |
| O My COD Delivery's Ltd | Hospitality | 2025 |
| O My COD Ltd | Hospitality | 2024 |
| O&S (Cardiff) Limited | Extraterritorial | 2020 |
| O'Connell's Lettings Ltd | Real estate | 2024 |
| O'Neill's Facilities & Waste Management Limited | Administrative & support | 2017 |
| O'Neill's FWM Employee Ownership Trustee Limited | Professional & scientific | 2024 |
| Oak Tree Energy Limited | Construction | 2021 |
| Oak Tree Farm (Pendoylan) Ltd | Financial services | 2019 |
| Oaka Vending (South Wales) Ltd | Wholesale & retail | 2023 |
| Oakbank Limited | Professional & scientific | 2008 |
| Oakclay Limited | Construction | 2025 |
| Oakdale Haulage Limited | Construction | 1998 |
| Oakfield Roofing Limited | Construction | 2020 |
| Oakland Partnership Development LLP | | 2015 |
| Oakover Limited | Extraterritorial | 2025 |
| Oakridge Aesthetics Ltd | Health & social care | 2023 |
| Oakvale Pine Ltd | Wholesale & retail | 2018 |
| Oakwoodupvc Ltd | Manufacturing | 2025 |
| Oakwynn Limited | Professional & scientific | 2019 |
| OBS Consulting Ltd | Information & communication | 2021 |
| Obsessed Beauty NLT Limited | Other services | 2019 |
| Occultatum Unus Ltd | Real estate | 2019 |
| Occurin Ltd | Information & communication | 2025 |
| Ocean Homes Development Limited | Households | 2005 |
| Ocean Signs (UK) Limited | Professional & scientific | 2009 |
| OD Bridgend Limited | Hospitality | 2018 |
| Odessey Services Limited | Professional & scientific | 2009 |
| ODT Groundworks Ltd | Construction | 2021 |
| OFF Grid 31 Ltd | Hospitality | 2023 |
| Ogmore Vale Bowls Ltd | Arts & recreation | 2020 |
| Ogmore Vale RFC Limited | Arts & recreation | 2021 |
| Ogmore Valley Building Supplies Limited | Wholesale & retail | 2001 |
| Ogmore Valley Dragons Boxing Club Ltd | Arts & recreation | 2012 |
| Ogmore Valley Store Ltd | Wholesale & retail | 2021 |
| Ogmore Valley Suicide Awareness CIC | Education | 2022 |
| OH SO Chic - Homeware & Gifts Ltd | Wholesale & retail | 2026 |
| OH SO Chic Ltd | Wholesale & retail | 2025 |
| OH Talent Solutions Ltd | Administrative & support | 2019 |
| Ohlala Shop Ltd | Wholesale & retail | 2014 |
| OJW Clothing Ltd | Wholesale & retail | 2023 |
| OK Environmental Services Limited | Administrative & support | 2010 |
| Old Faithful Ltd | Wholesale & retail | 2014 |
| Old Industries Limited | Hospitality | 2014 |
| Old Monk RUM Limited | Extraterritorial | 2010 |
| Oldamber Limited | Extraterritorial | 2025 |
| Oldbaron Limited | Wholesale & retail | 2024 |
| Oldgrant Limited | Extraterritorial | 2026 |
| Oleo Ltd | Wholesale & retail | 2020 |
| Oliver Huntley Limited | Real estate | 2016 |
| Olivia Marriott Properties Limited | Real estate | 2026 |
| Olivia's Muse Ltd | Professional & scientific | 2025 |
| Ollieolds Ltd | Real estate | 2025 |
| ON Stage Services Ltd | Information & communication | 2010 |
| ON TAP Services Limited | Administrative & support | 2017 |
| ON Target Tuition Ltd | Education | 2023 |
| ON Time Catering Ltd | Hospitality | 2024 |
| On-Site Welding and Engineering Company Limited | Wholesale & retail | 1956 |
| Onamika Ltd | Hospitality | 2025 |
| Onboard Sportswear Ltd | Manufacturing | 2020 |
| One 2 One Therapy Ltd | Health & social care | 2017 |
| One Post Production Ltd | Information & communication | 2018 |
| One Stop Motor Repairs Ogmore Vale Ltd | Wholesale & retail | 2024 |
| One Vision Digital Limited | Construction | 2003 |
| One Vision Photography Limited | Professional & scientific | 2016 |
| One Way Hire Limited | Transport & logistics | 2016 |
| ONE4 Consulting Ltd | Information & communication | 2022 |
| Oneocean Limited | Transport & logistics | 2010 |
| Onward 2 Upward Ltd | Other services | 2020 |
| Oooface Ltd | Wholesale & retail | 2022 |
| Oplearn Limited | Information & communication | 2017 |
| Optimus Digital LAB Limited | Health & social care | 2026 |
| Oracle Risk Management Ltd | Professional & scientific | 2017 |
| Orbis Employee Ownership Trustee Limited | Professional & scientific | 2023 |
| Orbis Environmental Ltd | Professional & scientific | 2016 |
| Orbit Home Improvements Limited | | 1981 |
| Orca Decorating & Fine Finishing Ltd | Construction | 2025 |
| Original Performance Motors Ltd | Wholesale & retail | 2023 |
| Orion Human Capital (HK) Limited | Financial services | 2023 |
| Orivo Properties Ltd | Households | 2025 |
| ORJ Ltd | Health & social care | 2020 |
| Orld Limited | Real estate | 2022 |
| Ormonvine Limited | Manufacturing | 1999 |
| Ortho-Clinical Diagnostics | Manufacturing | 1990 |
| OSC Data Centre Management Ltd | Professional & scientific | 2026 |
| Oskar Cleaning Solutions Limited | Administrative & support | 2026 |