| One Zero Energy Limited | Administrative & support | 2020 |
| ONE618 Financial Group Limited | Financial services | 2019 |
| Onebyte Ltd | Information & communication | 2005 |
| Onehaven Healthcare Limited | Health & social care | 2019 |
| Onelove United FC Sports Management Ltd | Education | 2025 |
| Oneonone Communications Ltd | Professional & scientific | 2010 |
| Oni's Medical Limited | Health & social care | 2022 |
| Oniric Limited | Arts & recreation | 2025 |
| Onju Limited | Hospitality | 2024 |
| Online Floors Ltd | Wholesale & retail | 2020 |
| Online Fundraising Limited | Information & communication | 2015 |
| Online Fusion Limited | Information & communication | 2010 |
| Online Giveaways Ltd | Wholesale & retail | 2021 |
| Online Tools UK Ltd | Wholesale & retail | 2024 |
| Only the Bald Ltd | Wholesale & retail | 2025 |
| Only Vans Parts, Sales and Services Ltd | Wholesale & retail | 2025 |
| Onoto England Limited | Extraterritorial | 2014 |
| Onpoint Design Ltd | Professional & scientific | 2025 |
| Onroute Marketing Ltd | Professional & scientific | 2022 |
| Onsite Systems Ltd | Extraterritorial | 2021 |
| Ontheclockprep Ltd | Wholesale & retail | 2025 |
| Onvi Tech Ltd | Information & communication | 2016 |
| Onward Enterprises Management Limited | Administrative & support | 2007 |
| Onyx PUB Company Limited | Hospitality | 2016 |
| Onyx RP Ltd | Arts & recreation | 2019 |
| Oovee Ltd | Information & communication | 2008 |
| Opal Futures Ltd | Information & communication | 2025 |
| Opale Management Services Limited | Professional & scientific | 2010 |
| Opblazen House Ltd | Construction | 2020 |
| Opcon JFK Ltd | Professional & scientific | 2024 |
| Opel Products Ltd | Wholesale & retail | 2025 |
| Open Door Theatre Productions Ltd | Arts & recreation | 2025 |
| Open Ergonomics Limited | Information & communication | 1992 |
| Open Heart Care Ltd | Professional & scientific | 2016 |
| Open Hill Limited | Manufacturing | 2018 |
| Open Quantum Limited | Information & communication | 2023 |
| Open Trade Network | Information & communication | 2015 |
| Open World Travel Limited | Administrative & support | 2006 |
| Opencoms Ltd | Information & communication | 2024 |
| Opencontact Limited | Administrative & support | 2009 |
| Opencontact Services Limited | Administrative & support | 2000 |
| Opengate Solutions Limited | Financial services | 2021 |
| Openpurpose Property Management Limited | Households | 1996 |
| Operation Properties Limited | Real estate | 2011 |
| Operator Licence Assist Ltd | Professional & scientific | 2014 |
| Opergy Finance Limited | Professional & scientific | 2020 |
| Opergy Group Ltd | Financial services | 2022 |
| Opergy Holdings Limited | Financial services | 2022 |
| Opergy Limited | Administrative & support | 2018 |
| Opergy Ventures Ltd | Professional & scientific | 2024 |
| Opet Solutions Ltd | Administrative & support | 2024 |
| Opex Coventry Ltd | Arts & recreation | 2019 |
| Opine Contracting Limited | Construction | 2018 |
| Opsstrata Ltd | Professional & scientific | 2025 |
| Opticraft Ltd | Wholesale & retail | 2024 |
| Optify Invest Limited | Financial services | 2024 |
| Optima Canine Products Limited | Manufacturing | 2025 |
| Optimal Birth Ltd | Education | 2021 |
| Optimal Health Enterprises Limited | Wholesale & retail | 2024 |
| Optimise Media (Europe) Limited | Professional & scientific | 2012 |
| Optimise Media (UK) Limited | Information & communication | 2002 |
| Optimise Media Group Limited | Financial services | 2001 |
| Optimise Media Ltd | Professional & scientific | 2015 |
| Optimum Agency Ltd | Transport & logistics | 2010 |
| Optimum Industrial Limited | Wholesale & retail | 2006 |
| Optimum Metalworking Limited | Manufacturing | 2014 |
| Optimum Oils Limited | Wholesale & retail | 1997 |
| Optimum Patient Care Global Limited | Information & communication | 2005 |
| Optimum Patient Care Holdings Limited | Financial services | 2025 |
| Optimum Property Consortium Limited | Real estate | 2026 |
| Option Rackheath Ltd | Real estate | 2021 |
| Options Glazing Ltd | Manufacturing | 2015 |
| Optometric Practice Ltd | Health & social care | 2026 |
| Opulentus Inc Ltd | Other services | 2024 |
| OR Distributions Ltd | Wholesale & retail | 2026 |
| OR Psychology Ltd | Health & social care | 2022 |
| ORA Executive Search Limited | Administrative & support | 2026 |
| ORA Wellness Limited | Hospitality | 2024 |
| Oracore Limited | Information & communication | 2024 |
| Orange Blossom Cafe Ltd | Hospitality | 2025 |
| Orange Elephant Theatre Company | Arts & recreation | 2024 |
| Orange FOX Electrical Limited | Construction | 2008 |
| Orange FOX Holdings Limited | Financial services | 2016 |
| Orange Genie PAY Limited | Information & communication | 2018 |
| Orange Heating Supplies Limited | Construction | 2019 |
| Orange Ice Cream and Dessert Parlour Ltd | Hospitality | 2014 |
| Orange Phoenix Ltd | Professional & scientific | 2018 |
| Orange Tree Homes Ltd | Real estate | 2016 |
| Orangeville Property Services Limited | Real estate | 2026 |
| Orari Ltd | Professional & scientific | 2015 |
| ORASIS21 Ltd | Health & social care | 2021 |
| ORB Organic Hair Ltd | Other services | 2010 |
| Orbitil Holdings Limited | Professional & scientific | 2025 |
| Orbitil Limited | Information & communication | 2013 |
| Orchard Fruits Direct Limited | Wholesale & retail | 2009 |
| Orchard House Accountancy (Dormant) Ltd | Extraterritorial | 2023 |
| Orchestrate Design and Build Ltd | Construction | 2015 |
| Orchid (Norwich) Limited | Construction | 2015 |
| Orchid Payroll Services Limited | Extraterritorial | 2007 |
| Orchidale Limited | Other services | 2021 |