| Online Taxi Services Bristol Ltd | Information & communication | 2026 |
| Online Web Store Ltd | Professional & scientific | 2012 |
| ONLINEMARKET2005 Ltd | Wholesale & retail | 2025 |
| Only Beyond Ltd | Real estate | 2017 |
| Only Green Bristol Ltd | Hospitality | 2019 |
| Only Nine Media Limited | Administrative & support | 1985 |
| Only Orca Limited | Administrative & support | 2015 |
| Onlyou Bristol Ltd | Health & social care | 2025 |
| Onlytoolsandhouses Ltd | Manufacturing | 2026 |
| Onno Group Limited | Professional & scientific | 2013 |
| Onno Holdings Limited | Real estate | 2019 |
| Onpoint Suction Excavation Ltd | Administrative & support | 2024 |
| Onrai Services Ltd | Professional & scientific | 2021 |
| Onroute Truckstops Limited | Other services | 1992 |
| Onsecurity Technology Limited | Information & communication | 2022 |
| Onsite Asset Management Limited | Real estate | 2024 |
| Onte Digital Ltd | Professional & scientific | 2022 |
| Ontrack Infrastructure Solutions Limited | Construction | 2025 |
| Ontrack Vehicle Transport Ltd | Transport & logistics | 2025 |
| Ontrax Consulting Limited | Professional & scientific | 2023 |
| Ontrax Rentals UK Limited | Administrative & support | 2021 |
| Onuah Estates Limited | Real estate | 2021 |
| Onuah Healthcare Limited | Health & social care | 2021 |
| Onyxden Healthcare Solutions Limited | Transport & logistics | 2025 |
| OOH AR Ltd | Information & communication | 2024 |
| OOM Labs Ltd | Wholesale & retail | 2026 |
| Oomph Drinks Ltd | Manufacturing | 2022 |
| Ooshka Bay Ltd | Manufacturing | 2024 |
| Ooty Global Limited | Wholesale & retail | 2022 |
| Oowee Diner Bristol Editions Ltd | Hospitality | 2021 |
| Oowee Editions Brighton Ltd | Hospitality | 2020 |
| Oowee London Ltd | Hospitality | 2019 |
| Oowee Ltd | Hospitality | 2016 |
| Oowee V Ltd | Hospitality | 2021 |
| Ooweenorthstreet Ltd | Hospitality | 2017 |
| Ooweevbaldwin Ltd | Hospitality | 2018 |
| Ooweevbrixton Ltd | Hospitality | 2019 |
| OP Barroso Limited | Health & social care | 2018 |
| OP Property Limited | Real estate | 2016 |
| OPA Bath Limited | Hospitality | 2005 |
| Opal Cloud Ltd | Real estate | 2025 |
| Opalcrown Limited | Real estate | 1993 |
| Opalglebe Limited | Real estate | 1992 |
| Opaltec International Limited | Information & communication | 1998 |
| Opalton Limited | Financial services | 2003 |
| Opalwood Homes and Co Ltd | Real estate | 2025 |
| Opaquex Ltd | Financial services | 2025 |
| OPD Properties Ltd | Real estate | 2025 |
| Opelton Limited | Wholesale & retail | 2009 |
| Open 24 Seven Limited | Wholesale & retail | 2007 |
| Open Air Visuals Ltd | Manufacturing | 2023 |
| Open Bionics Ltd | Professional & scientific | 2014 |
| Open Connections Holdings Limited | Professional & scientific | 1999 |
| Open Connections Solutions Limited | Administrative & support | 1999 |
| Open Door Coaching & Consulting Ltd | Professional & scientific | 2023 |
| Open Hearts Homecare Ltd | Health & social care | 2025 |
| Open Heavens CV Ltd | Health & social care | 2013 |
| Open Minds Active C.I.C. | Arts & recreation | 2019 |
| Open Nest Property Limited | Real estate | 2025 |
| Open Ocean A@S Ltd | Transport & logistics | 2024 |
| Open Partnership Solutions Limited | Manufacturing | 2004 |
| Open Pathways Education Limited | Education | 2026 |
| Open Plan Living and Construction Limited | Construction | 2022 |
| Open Plan Property Ltd | Real estate | 2019 |
| Open Planet Community Interest Company | Information & communication | 2022 |
| Open Planet Studios Ltd | Administrative & support | 2025 |
| Open Properties UK Ltd | Construction | 2017 |
| Open Storage Ltd | Transport & logistics | 2025 |
| Open Tennis UK Limited | Arts & recreation | 2017 |
| Open Trustees Limited | Administrative & support | 1992 |
| Open UP Music | Information & communication | 2007 |
| Openbrix Ltd | Information & communication | 2018 |
| Openflow Global Tech Ltd | Information & communication | 2026 |
| Openmind Systems Ltd | Information & communication | 2026 |
| Openscapes Collaborative Ltd | Professional & scientific | 2025 |
| Openshare Limited | Households | 2003 |
| Opera Magic Ltd | Extraterritorial | 2010 |
| Operanda Ltd | Administrative & support | 2008 |
| Operation Christmas Hamper CIC | Hospitality | 2023 |
| Operative Cleaning Solutions Ltd | Administrative & support | 2025 |
| Operii Financial Planning LLP | | 2007 |
| Opex Group Ltd | Other services | 2021 |
| Opex Holdings Ltd | Real estate | 2016 |
| Opex Logistics Limited | Transport & logistics | 2003 |
| Opey DFD Limited | Health & social care | 2025 |
| Ophi BO Limited | Wholesale & retail | 2021 |
| Opima Limited | Wholesale & retail | 2026 |
| Opitus Ltd | Information & communication | 2011 |
| Oplan Office Furniture Limited | Wholesale & retail | 2015 |
| Opposable Games Limited | Information & communication | 2011 |
| Opposable Group Limited | Professional & scientific | 2015 |
| Opposable Thumb Theatre Ltd | Arts & recreation | 2022 |
| Opposable VR Limited | Information & communication | 2015 |
| Opricade Ltd | Information & communication | 2025 |
| OPS People Limited | Administrative & support | 2019 |
| Ops-Foods Ltd | Hospitality | 2025 |
| Opshra Lobby Ltd | Wholesale & retail | 2025 |
| Opsys Fire & Security Limited | Administrative & support | 2023 |
| Opta Management Ltd | Real estate | 2010 |
| Optic Motion Ltd. | Arts & recreation | 2017 |