| Olusmak Ltd | Other services | 2025 |
| OM International Group Ltd | Wholesale & retail | 2014 |
| Omega Homecare Ltd | Health & social care | 2023 |
| OMG TEN Year Anniversary Ltd | Information & communication | 2013 |
| Omnia Elite Services Ltd | Administrative & support | 2023 |
| Omniawine Ltd | Wholesale & retail | 2023 |
| Omniminds Digital Limited | Information & communication | 2025 |
| Omniscient HE UK Ltd | Education | 2023 |
| ON Point Heating Limited | Construction | 2017 |
| ON Time Shred Limited | Water & waste | 2017 |
| On-Board Engineer Ltd. | Education | 2016 |
| Onair Services (UK) Limited | Extraterritorial | 1998 |
| One Body LDN Ltd | Health & social care | 2018 |
| One Madeira Grove Maintenance Company Limited | Construction | 2002 |
| One New Dawn Rising Ltd | Education | 2026 |
| One Question Shootout Productions Ltd | Administrative & support | 2022 |
| One SE Group Ltd | Real estate | 2026 |
| One Wise MAC Limited | Wholesale & retail | 2012 |
| Onefist Ltd | Construction | 2018 |
| Onesixtyone Ltd | Professional & scientific | 2025 |
| Oneskript Ltd | Arts & recreation | 2023 |
| Ongar Retail Ltd | Wholesale & retail | 2023 |
| Online Planet Limited | Wholesale & retail | 2025 |
| Online Shopping Concierge Service Limited | Administrative & support | 2018 |
| Only Deals Ltd | Wholesale & retail | 2012 |
| Onpoint IT Solutions Ltd | Construction | 2014 |
| Onyema Arbitration Services Limited | Administrative & support | 2020 |
| Opara Consulting Limited | Administrative & support | 2010 |
| Open Box Productions Limited | Arts & recreation | 2006 |
| Opening Ceremony Limited | Professional & scientific | 2023 |
| Openway Trading Limited | Real estate | 2014 |
| Ophthalmic Pro Ltd | Professional & scientific | 2015 |
| Optima Performance Limited | Administrative & support | 2014 |
| Optimal Service Management Ltd | Professional & scientific | 2013 |
| Optimedix Ltd | Health & social care | 2024 |
| Optimise London Limited | Construction | 2021 |
| Optimze Limited | Construction | 2025 |
| Optiv Consultancy Limited | Administrative & support | 2012 |
| Opulent Property DEV Limited | Real estate | 2017 |
| OR Ventures Limited | Real estate | 2022 |
| Orange Leisure Limited | Construction | 1995 |
| Orangeland Limited | Real estate | 1998 |
| Orbital Digital Limited | Administrative & support | 2012 |
| Orbital Engineering Ltd | Professional & scientific | 2025 |
| Orbital Media Limited | Other services | 1994 |
| Orbital Plant Services Ltd | Construction | 2017 |
| Orbital Publishing Limited | Administrative & support | 2009 |
| Oremeji Inc Ltd | Professional & scientific | 2020 |
| Organic Mediterr. Ltd | Wholesale & retail | 2024 |
| Original People Ltd | Administrative & support | 2022 |
| Origins 1450 Limited | Financial services | 2022 |
| Origins 1450 Retail Sukuk Limited | Real estate | 2025 |
| Origins 1450 Services Limited | Professional & scientific | 2025 |
| Oris-Crown Cargo Logistics/Auto Merchants Limited | Wholesale & retail | 2019 |
| Oscar B Ltd | Wholesale & retail | 2021 |
| Oscby Ltd | Administrative & support | 2020 |
| Oscon Limited | Information & communication | 2021 |
| Oslien Ltd | Real estate | 2021 |
| OTL L14 Ltd | Real estate | 2025 |
| Otpd Ltd | Construction | 2021 |
| Otterburn Medical Development Limited | Real estate | 2026 |
| OUR Life Story Ltd | Other services | 2017 |
| Ourvertu Limited | Administrative & support | 2015 |
| Out-Back Catering Events Limited | Hospitality | 2024 |
| Outrageous Films Limited | Information & communication | 2026 |
| Outsource Financial Solutions Ltd | Professional & scientific | 2020 |
| OVC Global Holdings Ltd | Financial services | 2026 |
| Overheard Network Limited | Other services | 2019 |
| Overton Electrics Ltd | Construction | 2022 |
| Oxford Medical Technology Ltd | Professional & scientific | 1999 |
| Oxford Valuation Partners Limited | Administrative & support | 2015 |
| Ozzan Ltd | Education | 2021 |
| P & L Communications Limited | Construction | 1993 |
| P & S Gills Limited | Transport & logistics | 2018 |
| P B 007 Limited | Construction | 2024 |
| P Devane Construction Limited | Construction | 2017 |
| P M T 2000 Limited | Health & social care | 1999 |
| P R Live Limited | Administrative & support | 2001 |
| P Sanghi Ltd | Health & social care | 2016 |
| P.F. Ahern & Sons Limited | Professional & scientific | 1975 |
| P.F. Ahern (London) Limited | Transport & logistics | 1985 |
| P.J.C. (Leasing) Limited | Administrative & support | 1988 |
| P.M.L Georgiou Limited | Real estate | 2018 |
| P.S.C Property Investments Ltd | Real estate | 2012 |
| P83 Construction Ltd | Construction | 2023 |
| Pacco Consulting Limited | Information & communication | 2013 |
| Pace Technology Limited | Manufacturing | 1984 |
| Pacific Distributors Limited | Wholesale & retail | 2021 |
| Pacific Naval Register of London Ltd | Transport & logistics | 2002 |
| Padd Connect Limited | Real estate | 2016 |
| Paddys Adventures Ltd | Real estate | 2022 |
| Padel Corner Ltd | Wholesale & retail | 2021 |
| Padel Edition Ltd | Real estate | 2021 |
| Padel Master (Basildon) Ltd | Arts & recreation | 2026 |
| Padel on Ltd | Arts & recreation | 2025 |
| Padel Protein Limited | Wholesale & retail | 2026 |
| Paige Properties Ltd | Real estate | 2017 |
| Pairpartners Limited | Wholesale & retail | 2024 |
| Paisley Amber Developments Limited | Construction | 2016 |
| Pakbrit Ltd | Wholesale & retail | 2025 |