| Omar Yanni Ltd | Health & social care | 2013 |
| Omec Consumer Products Limited | Manufacturing | 2018 |
| Omec Holdings Limited | Financial services | 2015 |
| Omec House Ltd | Real estate | 2017 |
| Omec Learn Limited | Education | 2021 |
| Omec PC Limited | Information & communication | 2010 |
| Omega Protect Ltd | Administrative & support | 2012 |
| Omega Technology Limited | Information & communication | 2010 |
| Omkaar Estates Limited | Real estate | 2018 |
| Omkaar Systems Limited | Information & communication | 2017 |
| Omnitech Consultancy Ltd | Professional & scientific | 2018 |
| OMO Properties Ltd | Real estate | 2018 |
| ONB Limited | Health & social care | 2018 |
| One Call IT Limited | Information & communication | 2009 |
| One Hereford Limited | Other services | 2023 |
| One MAD World Limited | Information & communication | 2018 |
| One Ninety Nine One Limited | Arts & recreation | 2015 |
| One Pepys Street Management Company Limited | Households | 2002 |
| One Stop Motorist Centre Ltd. | Wholesale & retail | 2001 |
| ONE6 Consulting Limited | Professional & scientific | 2006 |
| Onebrick Property Ltd | Households | 2024 |
| Onestonet Ltd | Education | 2025 |
| Onetruck Ltd | Transport & logistics | 2014 |
| Onform Management Ltd | Professional & scientific | 2025 |
| Ongole Limited | Wholesale & retail | 2018 |
| Oniko Properties Limited | Real estate | 2025 |
| Onitek Solutions Limited | Information & communication | 2013 |
| Onix Investments UK Ltd | Financial services | 2017 |
| Online Cloud Accountants Ltd | Professional & scientific | 2020 |
| Online Company Info Limited | Information & communication | 2013 |
| Online Contractor Accountants Ltd | Professional & scientific | 2017 |
| Onmedia Group Ltd | Professional & scientific | 2010 |
| Onslow Inc. Ltd | Other services | 2017 |
| Onstruction Ltd | Construction | 2018 |
| Ontario Medical Ltd | Health & social care | 2026 |
| Ontime Traders Ltd | Wholesale & retail | 2023 |
| Open Access FND Ltd | Transport & logistics | 2024 |
| Open Asset Group Ltd | Real estate | 2019 |
| Open Space Design Limited | Administrative & support | 2005 |
| Operon Consulting Limited | Professional & scientific | 2015 |
| Ophthalmic Instrument Company Ltd | Administrative & support | 2010 |
| Optical Sensing Ltd | Professional & scientific | 2021 |
| Optikal (Potters Bar) Ltd | Health & social care | 2008 |
| Optikal (Properties) Ltd | Real estate | 2023 |
| Optikal (Southgate) Ltd | Health & social care | 2009 |
| Optikal (Temple Fortune) Ltd | Health & social care | 2012 |
| Optimised Technologies Limited | Administrative & support | 2016 |
| Optimum Business Support Limited | Administrative & support | 2017 |
| Optimum Care Medical Ltd | Health & social care | 2025 |
| Optio Investment Advisors Ltd | Professional & scientific | 2025 |
| Option Couriers LLP | | 2014 |
| Options Insight Ltd | Professional & scientific | 2018 |
| Optivity Limited | Information & communication | 1999 |
| Optumis Limited | Information & communication | 2009 |
| Opulize Global Limited | Financial services | 2026 |
| Opuluxe Properties Limited | Real estate | 2023 |
| OPUS/2 Design Limited | Professional & scientific | 2007 |
| Orange Music Electronic Company Limited | Manufacturing | 1976 |
| Orange Shop Ltd | Wholesale & retail | 2021 |
| Orban Builders Ltd | Construction | 2017 |
| Orbit Rise Limited | Information & communication | 2024 |
| Orchard Court (Portman Square) Limited | Households | 1983 |
| Orej Financial Consultants Limited | Financial services | 2020 |
| Organic Faith Limited | Other services | 2019 |
| ORGANISE4U Ltd | Professional & scientific | 2007 |
| Orgdus Limited | Administrative & support | 2025 |
| Origin 8 Technologies Limited | Information & communication | 2008 |
| Origina Tech Ltd | Information & communication | 2022 |
| Original Biryaniwala London Ltd | Hospitality | 2021 |
| Origo Holdings Limited | Financial services | 2015 |
| Orlando Designs Ltd | Real estate | 2026 |
| Orlando Stone Ltd | Construction | 2021 |
| Ornovia Limited | Information & communication | 2024 |
| Orrell Road Valeting Ltd | Administrative & support | 2024 |
| Orthoclinic Holding Ltd | Professional & scientific | 2024 |
| Orthokind Limited | Health & social care | 2010 |
| Oryii Limited | Manufacturing | 2019 |
| Osac Management Limited | Administrative & support | 2013 |
| Osborne Baldwin Limited | Financial services | 2013 |
| Oscar Events House Ltd | Arts & recreation | 2018 |
| Oscou Limited | Professional & scientific | 2006 |
| Osda Construction Ltd | Construction | 2022 |
| Oshka Limited | Information & communication | 2008 |
| Ostrich Industries Limited | Extraterritorial | 2000 |
| OT Horizons Limited | Health & social care | 2020 |
| Othernaut Limited | Other services | 2021 |
| Otopark Limited | Real estate | 2020 |
| Otsokpolo Ltd | Information & communication | 2015 |
| Out-Spoken Press CIC | Information & communication | 2026 |
| Outreach Care Services Limited | Health & social care | 2023 |
| Outrivals Limited | Manufacturing | 2017 |
| Outsiders Garden Rooms Ltd | Construction | 2020 |
| Outsource Accounts Ltd | Professional & scientific | 2019 |
| Outsource Payroll Limited | Extraterritorial | 2005 |
| Ouverture London Ltd | Wholesale & retail | 2024 |
| OVA Enterprises Ltd | Hospitality | 2020 |
| Oven Blitzer (East Anglia) Ltd | Other services | 2015 |
| Overbray Limited | Health & social care | 2021 |
| Overlord Noodle Ltd | Hospitality | 2025 |
| Overseas Group Ltd | Health & social care | 2025 |