| Okenz Homes Limited | Real estate | 2023 |
| Okunz Limited | Administrative & support | 2014 |
| Old Albert Road Methodist Church Management Company Limited | Households | 2013 |
| Old Apple Consulting Limited | Professional & scientific | 2010 |
| Old BEN Homes Limited | Households | 2009 |
| Old EMT Limited | Health & social care | 2023 |
| Old Key West Limited | Financial services | 2024 |
| Old Quarry Management Limited | Households | 2008 |
| Old Salterns Gate Management Company Limited | Households | 1998 |
| Old School Mews (IOW) Management Company Limited | Households | 2021 |
| Old Stone Mews Management Company Ltd | Households | 2006 |
| Oldchurch Property Management Ltd | Real estate | 2021 |
| Olive Tree J7 Ltd | Real estate | 2016 |
| Oliver Partners LLP | | 2018 |
| Olivia's Boutique Limited | Wholesale & retail | 2016 |
| Olliesdesigns Ltd | Wholesale & retail | 2025 |
| Olmec Connections Limited | Information & communication | 2015 |
| OLS Building Services Limited | Construction | 2017 |
| Olwyn Ltd | Information & communication | 2026 |
| OMG Catering Services Limited | Hospitality | 2022 |
| OMG Event Management Limited | Hospitality | 2017 |
| Omgk Limited | Real estate | 2019 |
| Omni Opulence Limited | Wholesale & retail | 2024 |
| Omnitheia Limited | Information & communication | 2025 |
| Omogolo Limited | Health & social care | 2015 |
| Omorfia Beauty Ltd | Other services | 2022 |
| ON the Go Rmaps Limited | Wholesale & retail | 2025 |
| Onaro PC Ltd | Health & social care | 2016 |
| One Dollar Shop Ltd | Wholesale & retail | 2020 |
| One Family Solutions Ltd | Administrative & support | 2024 |
| One Flux Limited | Hospitality | 2026 |
| One Four Seven SW Limited | Hospitality | 2015 |
| One Happy Yogi Ltd | Wholesale & retail | 2020 |
| One Holyrood Holdings Ltd | Professional & scientific | 2016 |
| One Holyrood Ltd | Administrative & support | 2014 |
| One Last Word Limited | Information & communication | 2024 |
| One Louder Studios Limited | Information & communication | 2024 |
| One Solution Builders Ltd | Construction | 2021 |
| One Splendid Day Ltd | Professional & scientific | 2022 |
| One Stop Tree&Landscapes Limited | Administrative & support | 2025 |
| One to SIX Cavalier Quay RTM Company Limited | Real estate | 2007 |
| One Wood Street Management Company Limited | Households | 2009 |
| ONE80 Darts UK Limited | Manufacturing | 2011 |
| Onebuild Property Ltd | Real estate | 2025 |
| Onebuild Renovations Limited | Construction | 2025 |
| Oneport Building Ltd | Construction | 2021 |
| Onichu Ltd | Arts & recreation | 2020 |
| Online Fixers Ltd | Professional & scientific | 2025 |
| Online Gems Ltd | Wholesale & retail | 2026 |
| ONLINE381 Ltd | Wholesale & retail | 2024 |
| Only People Matter Ltd | Health & social care | 2019 |
| Only-Pawz Ltd | Wholesale & retail | 2023 |
| ONR Steel Exports Limited | Other services | 2021 |
| Onsaleluxury Ltd | Wholesale & retail | 2025 |
| Onward Professional Services Ltd | Information & communication | 2021 |
| Onwards Adventure Vehicles Limited | Manufacturing | 2018 |
| Onyx Luxury Stone Ltd | Mining & quarrying | 2023 |
| Oomoja Ltd | Information & communication | 2009 |
| Opal Fire Solutions Ltd | Public administration | 2025 |
| OPC Law Limited | Professional & scientific | 2024 |
| Openstart Limited | Extraterritorial | 2001 |
| Operasee Limited | Professional & scientific | 2006 |
| Operasonic CYF | Arts & recreation | 2014 |
| Operate & Excavate Ltd | Water & waste | 2019 |
| Operati Limited | Information & communication | 2021 |
| OPI Consulting Ltd | Information & communication | 2011 |
| Optech Solutions Limited | Wholesale & retail | 2007 |
| Optical Services & Sales Limited | Information & communication | 1997 |
| Optical Thin Film Glass Coating Ltd | Professional & scientific | 2022 |
| Optical Warehouse Limited | Wholesale & retail | 2019 |
| Optikombi Ltd | Construction | 2012 |
| Optima Evolve Aesthetics Limited | Health & social care | 2024 |
| Optimal Resilience Limited | Professional & scientific | 2014 |
| Optimise Partner Ltd | Professional & scientific | 2023 |
| Optimum Ageing Limited | Health & social care | 2022 |
| Optus Estates Limited | Real estate | 2022 |
| Optus Healthcare Limited | Administrative & support | 2019 |
| Opulent Deals Ltd | Wholesale & retail | 2025 |
| Opus 1 Solutions Limited | Administrative & support | 2022 |
| Opus 3 Ltd | Information & communication | 2022 |
| Orange Automotive Ltd | Wholesale & retail | 2013 |
| Orangepeel Creative Juice Ltd | Professional & scientific | 2014 |
| Orangutrans Limited | Administrative & support | 2010 |
| Orbis Road Solutions Limited | Transport & logistics | 2023 |
| Orbitgrade Ltd | Manufacturing | 2026 |
| Orchard (Newport) Management Company Limited | Households | 1986 |
| Orchard Properties Consultancy Limited | Extraterritorial | 2005 |
| Orchardcroft Management Limited | Households | 2019 |
| ORE Restaurants Limited | Hospitality | 2019 |
| Oriel Green Properties Limited | Real estate | 2021 |
| Oriental WOK Ltd | Hospitality | 2023 |
| Origami Marketing Limited | Professional & scientific | 2025 |
| Origin Japan Ltd | Wholesale & retail | 2024 |
| Origin Landscapes and Fencing Ltd | Administrative & support | 2024 |
| Originate Books Ltd | Information & communication | 2023 |
| Originate Film Ltd | Information & communication | 2023 |
| Originate Literary Agency Ltd | Information & communication | 2023 |
| Orion Scott Ltd | Real estate | 2019 |
| Oritor Services Ltd | Information & communication | 2005 |
| Oritor Solutions Limited | Information & communication | 2020 |