| Onmin Health Care Ltd | Health & social care | 2023 |
| ONS Incorporated Limited | Information & communication | 2005 |
| Onsite Labour Supply Limited | Administrative & support | 2025 |
| Ontimedelivery Courier Ltd | Transport & logistics | 2022 |
| Onward Finance Limited | Financial services | 2024 |
| Onyt Limited | Financial services | 2021 |
| Onyx Basement Services Ltd | Construction | 2021 |
| Opal & Ivory Properties Ltd | Real estate | 2024 |
| Opal Stay Solutions Ltd | Administrative & support | 2025 |
| Open Storage Solutions Ltd | Other services | 2024 |
| Openaudience Events Limited | Administrative & support | 2019 |
| Openaudience Ltd | Professional & scientific | 2010 |
| Opendoor XYZ CIC | Administrative & support | 2025 |
| Opensurveycraft Ltd | Information & communication | 2025 |
| Openview Document Management Solutions Limited | Administrative & support | 2015 |
| Opspot Ltd | Information & communication | 2021 |
| Optana Limited | Wholesale & retail | 2025 |
| Optec E&R Services Ltd. | Construction | 2015 |
| Optec Group Ltd | Energy | 2022 |
| Optifull Ltd | Information & communication | 2020 |
| Optima Attachment Training Ltd | Education | 2026 |
| Optimezz Ltd | Construction | 2025 |
| Optimum Construction & Interior Design Ltd | Construction | 2012 |
| Optiven UK Limited | Real estate | 2024 |
| Optomise Solutions Ltd | Public administration | 2024 |
| Opulence Properties Ltd | Households | 2021 |
| Opus Consulting Group Ltd | Information & communication | 2020 |
| Oracle Services Limited | Real estate | 1995 |
| Orange Eats Ltd | Hospitality | 2026 |
| Orange Genie Admin Limited | Administrative & support | 2006 |
| Orange Genie Compliance Ltd | Administrative & support | 2005 |
| Orange Genie Cover Limited | Administrative & support | 2003 |
| Orange Genie Group Limited | Professional & scientific | 2005 |
| Orange Gerbera Ltd | Other services | 2014 |
| Oranka Fruit Juices Limited | Wholesale & retail | 2002 |
| Orastar Consultants Ltd | Information & communication | 2023 |
| Oratory Padel Limited | Arts & recreation | 2025 |
| ORB Ventures Limited | Professional & scientific | 2023 |
| Orbit Strategy Ltd | Professional & scientific | 2013 |
| Orbita Consultancy Limited | Professional & scientific | 2011 |
| Orbital Business Services Limited | Professional & scientific | 2001 |
| Orca Property Lettings Ltd | Real estate | 2020 |
| Orchard Hill Machinery Ltd | Wholesale & retail | 2017 |
| Orchard Hill Machinery Sales Limited | Wholesale & retail | 2019 |
| Orchard Lane Capital Ltd | Financial services | 2025 |
| Orchard View Farm Limited | Wholesale & retail | 2022 |
| Ordinary Interiors Ltd | Professional & scientific | 2023 |
| Organic Depot Ltd | Wholesale & retail | 2025 |
| Orgshakers Limited | Professional & scientific | 2020 |
| Original Thinkers Limited | Professional & scientific | 2025 |
| Orion Associates Limited | Administrative & support | 2011 |
| Orion Electrotech Limited | Administrative & support | 1997 |
| Orion Managed Services Limited | Financial services | 2014 |
| Orlando's Fine Foods Ltd | Wholesale & retail | 2020 |
| Orpheus Limited | Real estate | 2003 |
| Orsiran Limited | Information & communication | 2022 |
| Ortc Limited | Arts & recreation | 2020 |
| Orthodontic Care Limited | Health & social care | 2014 |
| Orthopure Ltd | Manufacturing | 2017 |
| OS Engineering Limited | Construction | 2025 |
| Osaya Holdings Ltd | Real estate | 2024 |
| Osborn Heating & Plumbing Limited | Construction | 2022 |
| Osborne Management Services Limited | Professional & scientific | 2025 |
| Osimo Limited | Administrative & support | 2025 |
| Ostad Chauffeurs Ltd | Transport & logistics | 2025 |
| Ostara HR Ltd | Professional & scientific | 2025 |
| Ostebow Studios Ltd | Real estate | 2023 |
| OT Country Enterprises Limited | Arts & recreation | 2013 |
| Otmoor Electrical Limited | Construction | 2002 |
| Otmoor Plumbing & Heating Limited | Construction | 2009 |
| OTR Car Tours Ltd | Administrative & support | 2023 |
| Ottery Project Management Limited | Administrative & support | 2009 |
| OUR Laundry Hub Ltd | Other services | 2014 |
| Ourspace Childcare Limited | Health & social care | 2020 |
| OUT of the Ground Ltd | Administrative & support | 2021 |
| OUT the Box Speakers Ltd | Education | 2021 |
| Outline Cosmetics Ltd | Wholesale & retail | 2019 |
| Outofbarbarasbox Ltd | Professional & scientific | 2018 |
| Output Hub Solutions Limited | Information & communication | 2025 |
| Outright Building Services Ltd | Construction | 2025 |
| Outsource Support Limited | Information & communication | 2004 |
| Oval Autos Ltd | Wholesale & retail | 2014 |
| Ovalkey Ltd | Information & communication | 2011 |
| Oven Loving Leighton Buzzard Limited | Administrative & support | 2024 |
| Oven Loving Limited | Manufacturing | 2022 |
| Ovenman Limited | Administrative & support | 2006 |
| Overload Physiques Ltd | Other services | 2020 |
| Overwatch Network Services Ltd | Information & communication | 2015 |
| OVI & Deni Ltd | Transport & logistics | 2024 |
| Owais Raja Limited | Real estate | 2020 |
| Owen Price Associates Limited | Wholesale & retail | 2005 |
| Owens Advisory Ltd | Professional & scientific | 2019 |
| OWL Guardian Services Limited | Professional & scientific | 2012 |
| Owlfly Sulphur Ltd | Information & communication | 2023 |
| Own Ltd | Information & communication | 2015 |
| OWS Construction Ltd | Construction | 2022 |
| OX Architectural Limited | Other services | 2022 |
| OX Hardwood Limited | Manufacturing | 2022 |
| OX4 Pickups Limited | Manufacturing | 2025 |
| Oxbury Partners Ltd | Construction | 2020 |