| NT Framing Ltd | Construction | 2006 |
| Ntech Mechanical Solutions Limited | Professional & scientific | 2021 |
| NTS Events Ltd | Hospitality | 2017 |
| NU Ideas & Innovations Ltd | Wholesale & retail | 2021 |
| Nuanced Products Ltd | Professional & scientific | 2025 |
| NUC Z Limited | Transport & logistics | 2025 |
| Nucleus Financial Services Group Ltd | Professional & scientific | 2025 |
| Nugit Digital Services Limited | Information & communication | 2026 |
| NUH Rezan Ltd | Hospitality | 2019 |
| Number 9 Group Commercial Ltd | Wholesale & retail | 2016 |
| Number 9 Group Ltd | Professional & scientific | 2016 |
| Numeris Footwear Limited | Wholesale & retail | 2021 |
| Numerus Justus Ltd | Professional & scientific | 2026 |
| Numis Property Investment Limited | Administrative & support | 2025 |
| Nupropco Ltd | Real estate | 2017 |
| NUR Jewellery Ltd | Wholesale & retail | 2024 |
| Nurse & Payne Limited | Professional & scientific | 1991 |
| Nurturelab Education Ltd | Education | 2024 |
| Nutraceutical Biosciences Limited | Hospitality | 2020 |
| Nutri Stahl Limited | Wholesale & retail | 2001 |
| Nutrition IQ Ltd | Wholesale & retail | 2025 |
| Nuvive Construction Ltd | Construction | 2025 |
| Nuvive Property.Services Ltd | Construction | 2025 |
| Nwroc Limited Limited | Other services | 2020 |
| Nyddfwchgis Limited | Professional & scientific | 2019 |
| Nylon Fasteners Limited | Wholesale & retail | 2009 |
| Nylonz Ltd | Wholesale & retail | 2006 |
| Nyminute Ltd | Manufacturing | 2018 |
| O'Brien Security Systems Limited | Construction | 2004 |
| O'Brien's Electrical Services SW Limited | Construction | 2023 |
| O'Connell Holdings Ltd | Other services | 2016 |
| O'Connor & Davies Limited | | 1949 |
| O'Mara Fishing Ltd | Agriculture & fishing | 2024 |
| O'Sullivan's Riverbank Limited | Hospitality | 2018 |
| O'Sullivans Joinery Limited | Manufacturing | 2025 |
| O.E.M Swansea Ltd | Manufacturing | 2018 |
| OAD Concept Ltd | Manufacturing | 2025 |
| Oak & Stone Building Ltd | Construction | 2018 |
| Oak Tree Business Solutions Limited | Professional & scientific | 2024 |
| Oak Tree Food Group Limited | Wholesale & retail | 2019 |
| Oakdene Finance Limited | Financial services | 1995 |
| Oakfield Fifteen Holdings Limited | Financial services | 2026 |
| Oakly Ltd | Hospitality | 2012 |
| Oakmont Supply & Services Ltd | Hospitality | 2024 |
| Oaks and Brambles Limited | Other services | 2023 |
| Oaktree Construction (Wales) Limited | Construction | 2008 |
| Oaktree Parc Clinic (Swansea) Limited | Health & social care | 2018 |
| Oaktree Workshop Limited | Manufacturing | 2023 |
| Oakwood Brain Injury Case Management Limited | Health & social care | 2004 |
| Oasis (Swansea) Limited | Hospitality | 2015 |
| Oasis Dental Ltd | Health & social care | 2024 |
| Oasis Grillhouse Ltd | Hospitality | 2025 |
| Oasis Kebab House Ltd | Hospitality | 2025 |
| Obot Services Ltd | Wholesale & retail | 2025 |
| Observatory Property Ltd | Construction | 2026 |
| OC Carpentry Ltd | Construction | 2018 |
| OC Hire Ltd | Administrative & support | 2026 |
| OCB Goods Co. Ltd | Wholesale & retail | 2024 |
| Ocean Electrical and Property Services Ltd | Manufacturing | 2020 |
| Ocean Iris Limited | Hospitality | 2021 |
| Ocean Lilly Limited | Real estate | 2025 |
| Ocean View Gower Limited | Hospitality | 2011 |
| OCEAN11 Limited | Other services | 2019 |
| OCEAN6 Ltd. | Other services | 2015 |
| Oceancheer Limited | Arts & recreation | 1990 |
| OCI Consulting Limited | Wholesale & retail | 2005 |
| Ocon Group Limited | Construction | 2021 |
| Octopus 365 Ltd. | Information & communication | 2011 |
| ODD 3 Limited | Wholesale & retail | 2018 |
| ODD Outdoors Limited | Wholesale & retail | 2021 |
| ODD Pods Design Co. Ltd | Wholesale & retail | 2024 |
| Oddington Bees Ltd | Manufacturing | 2024 |
| Odysea (Europe) Ltd | Wholesale & retail | 2016 |
| Odysea (USA) Ltd | Wholesale & retail | 2016 |
| Odysea Global Ltd | Administrative & support | 2016 |
| Odyssey Training and Consultancy Limited | Transport & logistics | 2025 |
| OE Property Developments Ltd | Real estate | 2023 |
| OEA Foods Ltd | Wholesale & retail | 2020 |
| OFF Road Wales Limited | Agriculture & fishing | 2005 |
| Offbeat Holdings Ltd | Professional & scientific | 2021 |
| Offgrid Pizza and Sons Limited | Hospitality | 2025 |
| Office Revolt Limited | Professional & scientific | 2008 |
| Officium Hospitality Limited | Hospitality | 2025 |
| OG Property Services Ltd | Real estate | 2025 |
| OGN Consultancy Ltd | Professional & scientific | 2022 |
| OGW Gower Ltd | Agriculture & fishing | 2020 |
| OH Molly Aesthetics Ltd | Health & social care | 2024 |
| Ohana Management Ltd | Professional & scientific | 2024 |
| Ohmygames Ltd | Information & communication | 2025 |
| OHT Buildings Ltd | Construction | 2018 |
| OIL Services Ltd | Transport & logistics | 2019 |
| Oilgasprocedures Limited | Other services | 2017 |
| OJS Wine & Grocery Store Ltd | Wholesale & retail | 2026 |
| Old Farm Close Management Company Limited | Real estate | 2020 |
| Old School Nursery Landscaping Limited | Administrative & support | 2019 |
| Old School Place Ltd | Administrative & support | 2018 |
| Oldb Limited | Wholesale & retail | 2025 |
| Oldmills Antiques Ltd | Wholesale & retail | 2025 |
| Oldwalls Gower Limited | Hospitality | 2012 |
| Oldway Estates Ltd. | Real estate | 2000 |