| Ospreys in the Community | Arts & recreation | 2015 |
| Ospreys Rugby Limited | Arts & recreation | 2003 |
| Ossense Ltd | Manufacturing | 2023 |
| Ostc Wales LLP | | 2012 |
| Osteo Plus Limited | Manufacturing | 2009 |
| Ostrich Homes Ltd | Real estate | 2022 |
| OSW Scooters Limited | Wholesale & retail | 2017 |
| OT Legal Ltd | Administrative & support | 2025 |
| Otaku Venues Ltd | Wholesale & retail | 2025 |
| Ottalankal Ltd | Administrative & support | 2010 |
| Ouma Group Ltd | Professional & scientific | 2019 |
| Ouma Ltd | Administrative & support | 2017 |
| OUR Health Hub Ltd | Real estate | 2018 |
| OUR Health Pharma Investments Limited | Real estate | 2021 |
| OUR Health Pharma Limited | Wholesale & retail | 2019 |
| OUR Imagination IS NOT Limited | Extraterritorial | 2013 |
| OUR Lobular Breast Cancer Mission CIC | Other services | 2026 |
| OUR Place Limited | Health & social care | 2015 |
| OUT Loud Arts Collective CIC | Education | 2025 |
| OUT OUT Hospitality Ltd | Administrative & support | 2020 |
| Outdoor Welsh Events Limited | Administrative & support | 2003 |
| Outlier Learning Ltd | Professional & scientific | 2024 |
| Outside-In Systems Ltd | Construction | 2011 |
| Outsource Wales Limited | Information & communication | 2008 |
| Outventure Ltd | Wholesale & retail | 2021 |
| Outward Mindset Limited | Health & social care | 2026 |
| Overdrake Limited | Wholesale & retail | 1991 |
| Overland Autos PVT Limited | Wholesale & retail | 2023 |
| Ovicom Ltd | Wholesale & retail | 2013 |
| Owan East Limited | Wholesale & retail | 2025 |
| Owen John & Co. Ltd | Real estate | 2004 |
| Owens Haulage Limited | Transport & logistics | 2021 |
| OWL Plumbing and Heating Services Ltd | Construction | 2024 |
| Own the Process Ltd | Administrative & support | 2020 |
| OX Sportwear Limited | Wholesale & retail | 2024 |
| Oxford Care Holdings Ltd | Financial services | 2022 |
| Oxford Carpets Limited | Wholesale & retail | 2009 |
| Oxford Diecast Limited | Manufacturing | 1993 |
| Oxford Medical Consulting Limited | Health & social care | 2021 |
| Oxford Phones Ltd | Manufacturing | 2019 |
| Oxford Portland Limited | Construction | 2020 |
| Oxford Street Cafe Swansea Ltd | Hospitality | 2025 |
| Oxidane Compliance Ltd | Water & waste | 2023 |
| Oxoco Ltd | Construction | 2021 |
| Oxwich Coal House Limited | Real estate | 2012 |
| Oxwich Leisure Park Limited | Real estate | 2011 |
| Oxwich Management Company Limited | Administrative & support | 1998 |
| Oxwich Watersports Ltd | Arts & recreation | 2016 |
| Oyin Hair &Beauty Treatment Limited | Other services | 2025 |
| Oyster Cabs Limited | Transport & logistics | 2000 |
| Oyster Coffee Ltd | Hospitality | 2025 |
| Oyster Court Mumbles Management Limited | Households | 2019 |
| Oyster Gallery Ltd | Wholesale & retail | 2006 |
| Oyster Landscaping Limited | Administrative & support | 2023 |
| Oyster Property Ltd | Real estate | 2022 |
| Oysterhaus Limited | Real estate | 2021 |
| Oystermouth Court (Management) Limited | Households | 1984 |
| Oystermouth Living Limited | Health & social care | 2021 |
| Oystermouth Road Limited | Hospitality | 2020 |
| OZ Pictures Ltd | Information & communication | 2026 |
| O’Kelly Properties Ltd | Real estate | 2019 |
| P & G Pizza Ltd | Hospitality | 2023 |
| P & H Estates Ltd | Real estate | 2019 |
| P & R Garage Services Limited | Wholesale & retail | 2021 |
| P & T Logistics Ltd | Transport & logistics | 2022 |
| P C Spencer Contractors Limited | Construction | 2013 |
| P Collier Building Ltd | Construction | 2019 |
| P D Rees & SON Limited | Transport & logistics | 1988 |
| P J Booth Limited Liability Partnership | | 2002 |
| P J Construction Services Limited | Construction | 2015 |
| P R Beynon (Contractors) Limited | Construction | 2012 |
| P R Construction (South) Ltd | Construction | 2016 |
| P S Foley Optical Ltd | Wholesale & retail | 2024 |
| P&A Wellness Ltd | Other services | 2024 |
| P&C Property Maintenance Ltd | Construction | 2023 |
| P&Cm Thomas Ltd | Wholesale & retail | 2023 |
| P&M Bars and Restaurants Limited | Hospitality | 2024 |
| P&M Ecom Retail Ltd | Wholesale & retail | 2023 |
| P&R Housing Ltd | Real estate | 2026 |
| P&T Asset Management Ltd | Financial services | 2025 |
| P&W Property Limited | Real estate | 2021 |
| P-Matrix Ltd | Information & communication | 2011 |
| P. &. P. Fire and Fabrication Ltd. | Manufacturing | 1997 |
| P. Beynon Electrical Services Ltd | Energy | 2006 |
| P. Griffiths Pharmacy Ltd | Wholesale & retail | 1999 |
| P. Pugh (Builders) Limited | Construction | 2003 |
| P.A.J. Industrial Roofing Ltd. | Construction | 2001 |
| P.B Trade Car Sales Ltd | Wholesale & retail | 2017 |
| P.C. Jenkins Limited | Construction | 1976 |
| P.E.D Estates Limited | Financial services | 2025 |
| P.J. Jones (Plumbing and Heating) Ltd | Construction | 1997 |
| P.L.J. & SON Specialised Mouldings Limited | Manufacturing | 1983 |
| P.V.P. Multimedia Limited | Manufacturing | 2009 |
| P.Y.A. Developments Limited | Transport & logistics | 1996 |
| P2P Construction Consultancy Ltd | Professional & scientific | 2025 |
| P2P Housing Limited | Health & social care | 2020 |
| Pablo Market Ltd | Wholesale & retail | 2026 |
| Packer Optical Ltd | Wholesale & retail | 2022 |
| Padded 'N' Proud Limited | Wholesale & retail | 2024 |
| Page Engineering Services Ltd | Construction | 2025 |