| Oriental Tasty Cuisine Ltd | Hospitality | 2020 |
| Oriental YUM YUM Ltd | Hospitality | 2025 |
| Orientals Art Private Limited | Wholesale & retail | 2019 |
| Origin Logistics GP Limited | Real estate | 2024 |
| Origin Logistics LP | | 2024 |
| Original Nutrition Ltd | Other services | 2022 |
| Original Voice Ltd | Wholesale & retail | 2024 |
| Origo Copia Ltd | Professional & scientific | 2026 |
| Orinex Enterprises Limited | Wholesale & retail | 2025 |
| Orion Technical Engineering Services Ltd | Manufacturing | 2020 |
| Orion Trent Holdings Limited | Extraterritorial | 1999 |
| Orion Trent Limited | Extraterritorial | 1988 |
| Orizis Ltd | Information & communication | 2023 |
| Orka Interior Ltd | Professional & scientific | 2020 |
| ORL Cyprus Ltd | Health & social care | 2024 |
| Orlg Limited | Hospitality | 2023 |
| Orme Property Management Ltd | Households | 2025 |
| Ornate Homes Limited | Real estate | 2020 |
| ORO Events Limited | Arts & recreation | 2016 |
| ORO Group Services Limited | Financial services | 2025 |
| ORO Security Limited | Administrative & support | 2022 |
| Orona Limited | Manufacturing | 1977 |
| Oroso Ltd | Wholesale & retail | 2024 |
| Orovia Group Limited | Information & communication | 2004 |
| Orovia Software Ltd | Information & communication | 2011 |
| Orphicly Ltd | Wholesale & retail | 2026 |
| ORS (Holdings) Ltd | Manufacturing | 2020 |
| ORS Digital Ltd | Professional & scientific | 2026 |
| ORS Electrical & Security Ltd | Construction | 2022 |
| Ortega Performance Ltd | Education | 2022 |
| Ortheia Limited | Professional & scientific | 2017 |
| Orthogen Ltd | Wholesale & retail | 2022 |
| Ortus Property Education Ltd | Education | 2024 |
| Ortus Property Ltd | Construction | 2025 |
| Orva Studio Limited | Information & communication | 2023 |
| Orvanta Commerce Ltd | Wholesale & retail | 2026 |
| OS Air Ltd | Manufacturing | 2025 |
| OS Creative Limited | Arts & recreation | 2020 |
| OS Systems Engineering Ltd | Administrative & support | 2022 |
| Osas Services Ltd | Wholesale & retail | 2019 |
| Osas Vision Music Ltd | Information & communication | 2022 |
| Osay Ltd | Other services | 2020 |
| OSB FX Limited | Financial services | 2021 |
| Osborn Steel Stockholders Limited | | 1911 |
| Osborne Court Limited | Households | 1982 |
| Osborne Psalter Limited | Hospitality | 2023 |
| Osbornes of Derbyshire Ltd. | Wholesale & retail | 2024 |
| Oscar Beds Limited | Professional & scientific | 2015 |
| Oscar Medical Ltd. | Administrative & support | 2013 |
| Oscar N. Ltd | Wholesale & retail | 2018 |
| Oscar Saurin Limited | Manufacturing | 2022 |
| Oscar Signature Properties Limited | Real estate | 2025 |
| Oscillate Productions Limited | Information & communication | 2015 |
| Osefro Ltd | Wholesale & retail | 2023 |
| Osei Europe Ltd | Wholesale & retail | 2020 |
| Oselux Analytics Limited | Professional & scientific | 2022 |
| Oserian (Enfield) Limited | Professional & scientific | 2017 |
| Oserian (Heathfield) Limited | Construction | 2021 |
| Oserian Developments LLP | | 2019 |
| Osewo Ltd | Transport & logistics | 2019 |
| Osey Limited | Health & social care | 2014 |
| Oshi Homes Ltd | Households | 2024 |
| OSIGWE2006 Tradings Limited | Real estate | 2021 |
| OSL Capital Ltd | Financial services | 2026 |
| OSL Consulting Limited | Extraterritorial | 2022 |
| OSL Cutting Technologies Limited | Manufacturing | 2001 |
| OSL Group Holdings Limited | Financial services | 2002 |
| OSM Estates Ltd | Real estate | 2025 |
| Osmawest Limited | Administrative & support | 1988 |
| Osmetis Limited | Professional & scientific | 2016 |
| Osmis Mentoring & Inclusion Services CIC | Health & social care | 2018 |
| Osprey Fine Art Limited | Administrative & support | 2004 |
| Osprey Law Limited | Professional & scientific | 2020 |
| OSS Engineering Ltd | Professional & scientific | 2017 |
| Osseous Healthcare Ltd | Health & social care | 2022 |
| Ossett Tyres & Autos Ltd | Wholesale & retail | 2024 |
| Ossila Ltd | Professional & scientific | 2009 |
| Ossiossi Ltd | Professional & scientific | 2024 |
| OT Filmworks Limited | Information & communication | 2026 |
| Otaberg Consulting Ltd | Professional & scientific | 2017 |
| OTC Direct Services Ltd | Health & social care | 2019 |
| Otem Media Ltd | Information & communication | 2026 |
| Othello Enterprises Limited | Professional & scientific | 1995 |
| Other Housing Ltd | Information & communication | 2014 |
| Otherton Flying Training Ltd | Education | 2026 |
| Otley Walker Lettings Limited | Real estate | 2006 |
| Otolith Collective Limited | Administrative & support | 2010 |
| OTT Hydromet Limited | Wholesale & retail | 1999 |
| Otterburn Health Care Limited | Health & social care | 2010 |
| Otto's Restaurant Ltd | Hospitality | 2026 |
| Ottomate Limited | Information & communication | 2013 |
| Otuk Training Ltd | Education | 2015 |
| OTW Rail Services Ltd | Transport & logistics | 2021 |
| Ouahib Hanane Ltd | Wholesale & retail | 2025 |
| Ouch Health Ltd | Information & communication | 2022 |
| Oughtibridge Inn Ltd | Professional & scientific | 2020 |
| Oughtibridge Properties Limited | Real estate | 2018 |
| OUR Care 24 Ltd | Health & social care | 2023 |
| OUR COW Molly Dairy Limited | Manufacturing | 2014 |
| OUR Kind Ltd | Professional & scientific | 2022 |