| Olex Developments Ltd | Real estate | 2024 |
| Olinka Rose Properties Ltd | Real estate | 2025 |
| Olive & Twist on the Go Ltd | Hospitality | 2025 |
| Olives & Oils Limited | Wholesale & retail | 2003 |
| Olivia Head Ltd | Other services | 2026 |
| Olivia's Beauty (Swansea) Limited | Other services | 2016 |
| Ollie Lister Limited | Construction | 2015 |
| Olsen Software Limited | Information & communication | 1997 |
| Olten Ground Rents Limited | Real estate | 2025 |
| Oluwumjay Enterprise Ltd | Wholesale & retail | 2024 |
| Olympus Leadership & Management Limited | Arts & recreation | 2001 |
| OMA Limited | Professional & scientific | 2001 |
| OMC 8 Ltd | Real estate | 2025 |
| Omega Timber Frame Ltd | Construction | 2023 |
| Omni Customs Products Ltd | Transport & logistics | 2019 |
| Omni Marketing Agency Ltd | Information & communication | 2015 |
| Omor Ltd | Hospitality | 2020 |
| ON the C Ltd | Information & communication | 2017 |
| Onchain Bureau Capital Limited | Information & communication | 2024 |
| One Choice Cover Ltd | Professional & scientific | 2026 |
| One Eighty Light Ltd | Professional & scientific | 2014 |
| One J-Shed Dining Limited | Hospitality | 2023 |
| One Spot Healthcare Ltd | Health & social care | 2023 |
| One Stop Cctv Solutions Ltd | Administrative & support | 2024 |
| One Stop Holdings Ltd | Financial services | 2026 |
| Oner Signs Limited | Professional & scientific | 2023 |
| Onesociety Training Club Ltd | Arts & recreation | 2025 |
| Onest Dating Ltd | Information & communication | 2022 |
| Onetelco Ltd | Information & communication | 2017 |
| ONI Electrical Ltd | Construction | 2019 |
| Onih Group Ltd | Manufacturing | 2025 |
| Onirion Ltd | Health & social care | 2012 |
| Online Shopping UN Limited | Extraterritorial | 2013 |
| Onlineuk Ltd | Information & communication | 2022 |
| Onlyvans Group Ltd | Transport & logistics | 2025 |
| Ontime Logistics Limited | Transport & logistics | 2023 |
| Onward Logistics Ltd | Transport & logistics | 2025 |
| Onyx-Gemino Ltd | Wholesale & retail | 2024 |
| Onyxx Barbershop Ltd | Other services | 2025 |
| Open Plan Construction Ltd | Construction | 2025 |
| Open Road Adventures Limited | Transport & logistics | 2025 |
| Open Sky Development Holdings Ltd | Construction | 2018 |
| Operait Ltd | Professional & scientific | 2025 |
| OPG Properties Limited | Real estate | 2019 |
| OPI Sales Ltd | Real estate | 2025 |
| Opia Photography Limited | Professional & scientific | 2024 |
| Opidis Limited | Education | 2003 |
| Opportunity 6 Limited | Manufacturing | 2004 |
| Opra Eastbourne Ltd | Hospitality | 2021 |
| Opsexcellence Support Solutions Limited | Professional & scientific | 2026 |
| Opsura Ltd | Information & communication | 2025 |
| Optical Group Ltd | Administrative & support | 2018 |
| Optima Space Ltd | Wholesale & retail | 2026 |
| Optimal Plumbing Heating & GAS Services Limited | Construction | 2025 |
| Optimesia Consulting Limited | Professional & scientific | 2006 |
| Optimise-Ai Ltd | Information & communication | 2023 |
| Optimum Consulting Ltd | Professional & scientific | 2021 |
| Optimus Trading Services Ltd | Wholesale & retail | 2018 |
| Optimus-Biz Ltd | Wholesale & retail | 2014 |
| Optivex Limited | Financial services | 2025 |
| Optwal Limited | Other services | 2012 |
| Opus BS Ltd | Professional & scientific | 2020 |
| Opus Portway Limited | Real estate | 2017 |
| Opus Procurement Services Ltd | Professional & scientific | 2021 |
| Orac Limited | Manufacturing | 2001 |
| Orange and Blue Creative Services Limited | Information & communication | 2014 |
| Orange Line Limited | Professional & scientific | 2013 |
| Orbit Core Digital Limited | Information & communication | 2026 |
| ORC Designware Ltd | Information & communication | 2020 |
| Orca Financial Management Ltd | Professional & scientific | 2017 |
| Orchard Coffee House Ltd | Hospitality | 2024 |
| Orchid Capital Investment Ltd | Real estate | 2025 |
| Orchid Dermatology Limited | Health & social care | 2024 |
| Orderguard Ltd | Information & communication | 2025 |
| Orderly Distribution Limited | Wholesale & retail | 2018 |
| Orebody Risks Limited | Professional & scientific | 2016 |
| Oreillys Barber / Unisex Limited | Other services | 2011 |
| Organa Ltd | Health & social care | 2010 |
| Organicks Ltd | Real estate | 2023 |
| Oriental Spa 2026 Ltd | Health & social care | 2026 |
| Oriental Spa Swansea 1028 Ltd | Health & social care | 2024 |
| Oriental Star Limited | Wholesale & retail | 2017 |
| Origin Fitness & Wellness Limited | Arts & recreation | 2018 |
| Original Distribution Limited | Wholesale & retail | 2013 |
| Original Turkish Limited | Hospitality | 2019 |
| Orlando Talent Solutions Limited | Other services | 2025 |
| Ortharize Limited | Administrative & support | 2019 |
| OS1 Limited | Administrative & support | 2019 |
| Osama Transport Ltd | Transport & logistics | 2020 |
| Osborn Pipe Fitting Services Limited | Construction | 2016 |
| Osborne Jones Logburners and Stoves Ltd | Construction | 2023 |
| Osca Design Ltd | Manufacturing | 2023 |
| Oscar's Barber NO 1 Limited | Other services | 2022 |
| Oscars Barber Ltd | Other services | 2025 |
| Osceazy Ltd | Education | 2025 |
| Oseng-Rees Reflection Limited | Manufacturing | 2022 |
| OSH 2026 Limited | Professional & scientific | 2026 |
| Osprey Engineering Inspection Limited | Professional & scientific | 2017 |
| Osprey Engineering Limited | Energy | 2021 |
| Osprey Marketing Systems Limited | Information & communication | 2000 |