| Openwork Independent Solutions Limited | Professional & scientific | 2010 |
| Openwork Limited | Financial services | 2002 |
| Openwork Partnership LLP | | 2005 |
| Openwork Services Limited | Other services | 1998 |
| Operation Baseline Limited | Professional & scientific | 2020 |
| Operation Fit Ltd | Arts & recreation | 2018 |
| OPS Façade Engineering Limited | Construction | 2018 |
| OPS Structures Ltd | Construction | 2014 |
| Opset Limited | Wholesale & retail | 2026 |
| Optgrpuk Ltd | Manufacturing | 2025 |
| Opticode Consultancy Ltd | Information & communication | 2025 |
| Optika Ltd | Health & social care | 2022 |
| Optiko Ltd | Public administration | 2024 |
| Optima 84 FS Ltd | Professional & scientific | 2025 |
| Optima Consulting & Advisory Group Limited | Extraterritorial | 2013 |
| Optima Defence Services Limited | Extraterritorial | 2013 |
| Optima Financial Solutions Group Limited | Professional & scientific | 2016 |
| Optima Intelligence Limited | Administrative & support | 2018 |
| Optima Motors Ltd | Wholesale & retail | 2004 |
| Optima Reconstruction Limited | Extraterritorial | 2013 |
| Optimal Land and New Homes Limited | Professional & scientific | 2025 |
| Optimised Waste Logistics Limited | Manufacturing | 2016 |
| Optimism Health Group Ltd | Health & social care | 2020 |
| Optimizon Limited | Wholesale & retail | 2016 |
| Optimum Professional Services Ltd | Financial services | 2017 |
| Optimus Concierge Limited | Other services | 2023 |
| Optional Ltd | Information & communication | 2020 |
| Options With Learning Limited | Other services | 2001 |
| Optipro Ltd | Health & social care | 2023 |
| Optivara Consultancy Limited | Professional & scientific | 2025 |
| Optix Europe Limited | Information & communication | 2024 |
| Optme-In Ltd | Construction | 2025 |
| Opus (SW) Holdings Limited | Financial services | 2025 |
| Opus Group (SW) Limited | Construction | 2025 |
| Opusxenta Limited | Information & communication | 2020 |
| ORA Sports Tech Ltd | Information & communication | 2026 |
| Orabek Limited | Health & social care | 2023 |
| Oracle Property Developers UK Ltd | Real estate | 2021 |
| Orancy Planet Ltd | Professional & scientific | 2021 |
| Orange Automation Limited | Administrative & support | 2013 |
| Orange Resourcing Limited | Administrative & support | 2013 |
| ORB Hairdressing Ltd | Other services | 2018 |
| Orbasli Property Holding Ltd | Financial services | 2023 |
| Orchard (Broome Manor) Residents Limited(The) | Other services | 1978 |
| Orchard Building Ltd | Construction | 2022 |
| Orchard Court (Swindon) Management Company Limited | Households | 1989 |
| Orchard Place (Swindon) Residents Management Company Limited | Administrative & support | 2005 |
| Orchard Press Holdings Limited | Professional & scientific | 2015 |
| Orchard Press Limited | Manufacturing | 1989 |
| Orford and Baker Ltd | Hospitality | 2021 |
| Organised Record Keeping Ltd | Administrative & support | 2016 |
| Origami Flow Ltd | Arts & recreation | 2025 |
| Origin Health and Safety Ltd | Professional & scientific | 2021 |
| Original Flavor Ltd | Hospitality | 2025 |
| Originzone Limited | Manufacturing | 2002 |
| Orion Autos Ltd | Wholesale & retail | 2026 |
| Ormal Electronics Limited | Manufacturing | 2001 |
| Oroi Limited | Professional & scientific | 2016 |
| Orpesa Money Transfer Limited | Financial services | 2023 |
| Orphea Care Services Limited | Health & social care | 2017 |
| Orvie RK Limited | Health & social care | 2015 |
| Oryx Digital Limited | Information & communication | 2005 |
| OS Optics Ltd | Health & social care | 2026 |
| Osaretin Limited | Public administration | 2023 |
| Osbond Media Limited | Information & communication | 2023 |
| Osborne Family Homes Limited | Real estate | 2025 |
| Osborne Inns Limited | Hospitality | 1991 |
| Oscar Grill Ltd | Hospitality | 2025 |
| Oscommerce Limited | Information & communication | 2021 |
| OSE Security Systems Ltd | Administrative & support | 2002 |
| Osikaba Ltd | Health & social care | 2022 |
| OSJ Cables & Networking Ltd | Manufacturing | 2022 |
| Ospreyraj BIM Management Ltd | Information & communication | 2023 |
| OSR Technology Ltd | Information & communication | 2015 |
| Ostara Talent Limited | Administrative & support | 2026 |
| Osvaldo Ltd | Transport & logistics | 2021 |
| OSX Ltd | Information & communication | 2003 |
| OT House Ltd | Information & communication | 2020 |
| OTE Swindon Anodising Limited | Professional & scientific | 2021 |
| Otilia Florence Ltd | Health & social care | 2025 |
| Otter Commercial Limited | Real estate | 2026 |
| Ottr Works Ltd | Information & communication | 2017 |
| Oumie Print Limited | Manufacturing | 2022 |
| Oupie & SON LLP | | 2025 |
| OUR Babylon Garden Ltd | Manufacturing | 2022 |
| OUR Inventions Limited | Information & communication | 2010 |
| Ourspace Haven CIC | Education | 2025 |
| OUT of the Box Catering Ltd | Hospitality | 2017 |
| Outbace Limited | Professional & scientific | 2020 |
| Outlet Foods Limited | Hospitality | 2022 |
| Outline by Turner & Turner Ltd | Professional & scientific | 2024 |
| Outlook Limited | Construction | 1992 |
| Outsearch Ltd | Administrative & support | 2016 |
| Outsideclinic JVP 01 Ltd | Health & social care | 2021 |
| Outsideclinic JVP 02 Ltd | Health & social care | 2021 |
| Outsideclinic JVP 03 Ltd | Health & social care | 2021 |
| Outsideclinic JVP 05 Ltd | Health & social care | 2022 |
| Outsideclinic JVP 06 Ltd | Health & social care | 2022 |
| Outsideclinic JVP 07 Ltd | Health & social care | 2022 |
| Outsideclinic JVP 08 Ltd | Health & social care | 2022 |