| Aprico Holdings Limited | Financial services | 2023 |
| Aprogo Limited | Extraterritorial | 2011 |
| Apsley House Consulting Ltd | Professional & scientific | 2025 |
| Apsley Technical Services Limited | Construction | 2016 |
| Apsley Trustees Limited | Professional & scientific | 2024 |
| Aqil Consultancy Ltd | Information & communication | 2024 |
| Aqson Construction Ltd | Construction | 2013 |
| Aqua Unicorn Ltd | Health & social care | 2025 |
| Aquarisus Ltd | Wholesale & retail | 2025 |
| Aquarius I T Solutions Limited | Information & communication | 2023 |
| Aquaverd Ltd | Wholesale & retail | 2026 |
| Aquizer Ltd | Information & communication | 2025 |
| AR Oxford Group Limited | Real estate | 2023 |
| AR2 Property Group Ltd | Construction | 2024 |
| ARA IT Consultancy Limited | Information & communication | 2014 |
| ARA Real Estate Limited | Information & communication | 2019 |
| Arabian MAU Rescue Ltd | Wholesale & retail | 2024 |
| Arajam Ltd | Financial services | 2026 |
| Arakan Solutions Ltd | Administrative & support | 2003 |
| Aramis Estate Holdings Limited | Administrative & support | 2018 |
| Aravax Limited | Professional & scientific | 2024 |
| ARB Industries Limited | Information & communication | 2012 |
| ARB Oxford Ltd | Information & communication | 2018 |
| Arbor Tree Specialists Ltd | Agriculture & fishing | 2017 |
| Arbuk Limited | Agriculture & fishing | 2015 |
| ARC Creative Engineering Ltd | Professional & scientific | 2026 |
| Arcangel Enterprises Limited | Professional & scientific | 2008 |
| Archangel Aerospace Ltd | Manufacturing | 2010 |
| Archaz Limited | Wholesale & retail | 2017 |
| Archie Cars Ltd | Administrative & support | 2026 |
| Architectural Design & Fabrication Limited | Manufacturing | 2009 |
| Architecture Landscape Urbanism Limited | Professional & scientific | 2013 |
| Archon Advisory Partners Ltd | Professional & scientific | 2025 |
| Archspire Limited | Professional & scientific | 2025 |
| Archway Care and Support Ltd | Health & social care | 2025 |
| Arctic Air (Mechanical) Limited | Construction | 2015 |
| Arcvolt Electrical Services Ltd | Construction | 2022 |
| Arcwind Limited | Information & communication | 1989 |
| Arcz Limited | Professional & scientific | 2025 |
| Arda BIO Consulting Ltd | Professional & scientific | 2025 |
| Ardant Projects Limited | Real estate | 1991 |
| Arden Media Limited | Other services | 2008 |
| Ardent Inventive Limited | Professional & scientific | 2013 |
| Ardor Media Ltd | Information & communication | 2025 |
| Area Eco C.I.C. | Professional & scientific | 2008 |
| Areliablecreative Limited | Professional & scientific | 2006 |
| Arena Ventures Limited | Professional & scientific | 2021 |
| Arfc Trading Ltd | Administrative & support | 2013 |
| ARG Consultancy Ltd | Health & social care | 2026 |
| Argonaut Therapeutics Limited | Professional & scientific | 2016 |
| Aria Estates Developments Limited | Construction | 2020 |
| Aria Estates Limited | Real estate | 2004 |
| Ariana Multiservice Housecare Limited | Construction | 2025 |
| Arin Corporations Ltd | Real estate | 2026 |
| Arin Investments Ltd | Real estate | 2026 |
| Aringrove Limited | | 1983 |
| Arise Smart Enterprises Ltd | Real estate | 2016 |
| Aristotle Lane Estate Company Limited | Households | 2012 |
| Aritech Solutions Ltd | Information & communication | 2021 |
| Arith Project Limited | Construction | 2023 |
| Arival Developments Ltd | Real estate | 2024 |
| ARK Future Solutions Limited | Professional & scientific | 2023 |
| ARK Malik Foods Limited | Hospitality | 2024 |
| ARK Media Inc Ltd | Professional & scientific | 2021 |
| ARK Metrica Ltd | Manufacturing | 2019 |
| ARK Scaffolding (Oxford) Limited | Construction | 2008 |
| Arlex Technology Limited | Information & communication | 2025 |
| Arlowe Drive Management Company Limited | Extraterritorial | 2010 |
| Armitage Design & Build Ltd | Construction | 2011 |
| Armstrong Close (Harwell) Limited | Households | 2021 |
| Aroul's Limited | Financial services | 2010 |
| Arrivalz Limited | Transport & logistics | 2023 |
| Arrize Xtech UK Limited | Financial services | 2022 |
| Arrow Public Relations Ltd | Professional & scientific | 2012 |
| Arroware Ltd | Information & communication | 2021 |
| Arrowfield Capital Nominees Ltd | Financial services | 2019 |
| Arrowhead Technology Limited | Information & communication | 2022 |
| Art Broker Finance Limited | Administrative & support | 2012 |
| Art Cafe EST. Limited | Hospitality | 2024 |
| Art Over There Ltd | Information & communication | 2017 |
| Art Spark Limited Partnership | | 1990 |
| Art-Sauce CIC | Education | 2022 |
| Artemida Ademaj Ltd | Real estate | 2024 |
| Arthropredictx Ltd | Extraterritorial | 2024 |
| Arthur Edward Associates Limited | Administrative & support | 2002 |
| Arthurs Waggly Tails Pet Services Limited | Wholesale & retail | 2023 |
| Artificial Grass Co Oxfordshire Ltd | Construction | 2020 |
| Arts at the Old Fire Station | Arts & recreation | 2010 |
| Artux Limited | Transport & logistics | 2020 |
| Aruk II Humanitarian Services (UK) C.I.C. | Professional & scientific | 2026 |
| Arundel Homes Ltd | Real estate | 2020 |
| Arundel Meadows Limited | Construction | 2017 |
| ARX Logic Ltd | Information & communication | 2020 |
| Aryana Communications Limited | Professional & scientific | 2017 |
| AS Divine Properties Ltd | Real estate | 2021 |
| AS Portfolio Limited | Real estate | 2023 |
| ASA Housing Limited | Real estate | 2023 |
| Asaker Anaesthetics Limited | Health & social care | 2026 |
| Asanr Limited | Administrative & support | 2022 |
| Asara Group Limited | Financial services | 2024 |