| Madley Park Hall Trust | Health & social care | 2011 |
| Madsdoa Limited | Health & social care | 2018 |
| MAE Way Limited | Professional & scientific | 2020 |
| Magdalen Medical Publishing Ltd | Extraterritorial | 2016 |
| Magic of Lapland Limited | Administrative & support | 2005 |
| Magic Sand Ltd | Arts & recreation | 2018 |
| Magnetic Pumping Solutions Limited | Manufacturing | 2014 |
| Magnezium Ltd | Arts & recreation | 2019 |
| Magnolia Music Limited | Information & communication | 2025 |
| Maher Consultancy Services Ltd | Professional & scientific | 2019 |
| Maidana Andrews Ltd | Real estate | 2013 |
| Main Trade Services Cleaning Ltd | Water & waste | 2021 |
| Main Trade Services Ltd | Construction | 2018 |
| Maison Clements Limited | Real estate | 2025 |
| Majenta PLM Ltd | Extraterritorial | 2006 |
| Malachite Masonry Ltd | Construction | 2024 |
| Malcolm Hawkins Ltd | Extraterritorial | 2024 |
| Malthouse Close Management Company Limited | Households | 2010 |
| Malweld Limited | Manufacturing | 2017 |
| Mangetout Oxford Ltd | Hospitality | 2015 |
| Manor Developments Oxford Limited | Construction | 1991 |
| Manor View (Steeple Claydon) Management Company Limited | Households | 2021 |
| Manorwood Consultancy Ltd | Professional & scientific | 2020 |
| Manorwood Homes Ltd | Extraterritorial | 2021 |
| Manumission Consultancy Ltd | Professional & scientific | 2017 |
| Mapfh Limited | Wholesale & retail | 2005 |
| Maps International Ltd. | Other services | 1996 |
| March Consultants Limited | Other services | 2017 |
| Marcus Griffith Design Limited | Professional & scientific | 2019 |
| Margaret L Johnson Holdings Limited | Real estate | 2018 |
| Margaret L Johnson Marlborough Limited | Real estate | 2018 |
| Marianta Ltd | Information & communication | 2016 |
| Marina Hewett Ltd | Professional & scientific | 2026 |
| Marina Way Management (Number Three) Company Limited | Households | 2002 |
| Marina Way Management (Number Two) Company Limited | Households | 2002 |
| Marina Way Management Company Limited | Households | 2001 |
| Maritrace Limited | Information & communication | 2012 |
| Marjan International UK Ltd | Wholesale & retail | 2023 |
| Mark Anderson Limited | Professional & scientific | 2009 |
| Mark Barnard Ideas Limited | Professional & scientific | 2011 |
| Mark Powell Mechanical Services Limited | Manufacturing | 2001 |
| Mark Stoneham Limited | Health & social care | 2009 |
| Mark Thornley Limited | Information & communication | 2014 |
| Mark Winks Limited | Extraterritorial | 2012 |
| Market Access Limited | Administrative & support | 2017 |
| Marketing With Meaning Ltd | Professional & scientific | 2023 |
| Marlake Landscaping Limited | Administrative & support | 2023 |
| Marlborough Place (Woodstock) Management Company Limited | Households | 2010 |
| Marlborough Place (Woodstock) Phase 2 Management Company Limited | Households | 2015 |
| Marldell Limited | Professional & scientific | 2014 |
| Marlev Ltd | Real estate | 2017 |
| Marmaduke's Property Limited | Real estate | 2015 |
| Marments Limited | Financial services | 1914 |
| Marmite Property’S Limited | Households | 2024 |
| Marquez Management Limited | Professional & scientific | 2024 |
| Marsh Lane (Didcot) Freehold Company Limited | Households | 2025 |
| Marsh Lane (Didcot) Management Company Limited | Households | 2010 |
| Marshalls Plumbing and Heating Engineers Ltd | Construction | 2021 |
| Martin Gates Publishing Limited | Information & communication | 1993 |
| Martin Spicer Consultancy Limited | Professional & scientific | 2016 |
| Martyn R COX Limited | Real estate | 2017 |
| Mary Ann's Recruitment Services Ltd. | Administrative & support | 2006 |
| Masala Girls Innovation Limited | Information & communication | 2024 |
| Maserule Ltd | Health & social care | 2018 |
| Masri & Sons Ltd | Hospitality | 2018 |
| Mass Impressions Ltd | Information & communication | 2011 |
| MATCH2LEARN Ltd | Manufacturing | 2024 |
| Mathews Comfort Equity Limited | Financial services | 2024 |
| Mathews Comfort Financial Services Limited | Financial services | 1987 |
| Mathews Comfort Group Limited | Administrative & support | 2018 |
| Mathews Comfort Holdings Limited | Administrative & support | 2000 |
| Mathews Comfort Wealth Management Limited | Extraterritorial | 2004 |
| Matt Bray Creative Limited | Information & communication | 2013 |
| Matt Patrick Limited | Administrative & support | 2012 |
| Matthew Carter Removals and Storage Ltd | Transport & logistics | 2012 |
| Matthew Collins Furniture Limited | Manufacturing | 2008 |
| Matthew Collins Property Limited | Administrative & support | 2014 |
| Matthew Ellett Ltd | Administrative & support | 2014 |
| Maundrell & Co (Supercars) Limited | Wholesale & retail | 2012 |
| Mavari Holdings Limited | Financial services | 2026 |
| Mavari Medical Limited | Wholesale & retail | 2026 |
| Maxfield Holdings Ltd | Real estate | 2022 |
| Maxroy Group Ltd | Wholesale & retail | 2022 |
| Maxward Technology Limited | Information & communication | 1998 |
| Mayarun Limited | Real estate | 2012 |
| Mayarun Properties Limited | Real estate | 2012 |
| Mayes Construction and Design Ltd | Construction | 2025 |
| Maylarch Environmental Limited | Construction | 1984 |
| Maylarch Limited | Professional & scientific | 2003 |
| Maylarch Recycling Limited | Water & waste | 2006 |
| Mbbh Holdings Limited | Financial services | 2018 |
| MC & P Consultants Limited | Information & communication | 2000 |
| MCA Chartered Landscape Architects Limited | Professional & scientific | 2003 |
| Mccormick + Smith Architects Limited | Professional & scientific | 2004 |
| Mccurdy Russell Limited | Professional & scientific | 2006 |
| Mcdtv Limited | Information & communication | 2015 |
| Mckenna Environmental Limited | Water & waste | 1964 |
| Mcmahon (UK) Limited | Professional & scientific | 2013 |
| Mcmahon Scaffolding and Rigging Ltd | Administrative & support | 2010 |
| Mcmahon Scaffolding Ltd | Professional & scientific | 2009 |