| Oma-Fix Ltd | Construction | 2020 |
| OMS Planning Consultants Ltd | Professional & scientific | 2021 |
| ON Point: Inspire Limited | Health & social care | 2022 |
| ON the Mend Ltd | Other services | 2016 |
| ON Wheelz Automotive Ltd | Wholesale & retail | 2023 |
| ON Your Mind Ltd | Health & social care | 2025 |
| On-Art Ltd | Other services | 2025 |
| Oncotails Ltd | Professional & scientific | 2025 |
| One Alliance Plant Hire Ltd | Administrative & support | 2025 |
| One Caribbean Cargo Ltd | Real estate | 2016 |
| One Shot Music Ltd | Wholesale & retail | 2010 |
| One Stop Building & Construction Ltd | Construction | 2023 |
| One Thousand and One Group Ltd | Health & social care | 2025 |
| One to One Personnel Limited | Administrative & support | 2004 |
| One Vision Developments Limited | Construction | 2024 |
| Oneclinic Leigh Ltd | Health & social care | 2023 |
| Oneill Contracting Limited | Construction | 2015 |
| Oneoftwo Ltd | Manufacturing | 2026 |
| Oneresponseuk Ltd | Other services | 2015 |
| Online Consultants International Limited | Professional & scientific | 1988 |
| Online House Plants Limited | Wholesale & retail | 2021 |
| Online Interiors Ltd | Construction | 2006 |
| Online Vinyl Ltd | Wholesale & retail | 2019 |
| Onyx Real Consulting Ltd | Financial services | 2022 |
| Opal Advantage Limited | Extraterritorial | 2018 |
| Opaline Experiences Ltd | Hospitality | 2025 |
| Open Heart Properties Limited | Real estate | 2019 |
| Openwood Aluminium Limited | Wholesale & retail | 2013 |
| Openwood Special Projects Limited | Construction | 2011 |
| Openwood Structural Glazing Limited | Wholesale & retail | 2013 |
| Openwood Windows & Doors Limited | Extraterritorial | 2016 |
| Opes MRF 2013 Limited | Water & waste | 2013 |
| OPT Enterprises Limited | Arts & recreation | 2017 |
| Optemal Ltd. | Information & communication | 2012 |
| Optical World Limited | Information & communication | 1971 |
| Opticom Solutions Ltd | Other services | 2009 |
| Optify Consulting Ltd | Information & communication | 2024 |
| Optimal Health & Wellness Limited | Health & social care | 2017 |
| Optimal HR Assist Limited | Administrative & support | 2026 |
| Oracle Hair Limited | Other services | 2025 |
| Oracle Images UK Ltd | Professional & scientific | 2008 |
| Orange Portals Ltd | Information & communication | 2020 |
| ORB Search Limited | Professional & scientific | 2018 |
| Orbital Project Management Limited | Professional & scientific | 2024 |
| ORE Estates Limited | Professional & scientific | 2021 |
| Oriental Lily 161 Ltd | Hospitality | 2024 |
| Oriental Lily 2026 Ltd | Hospitality | 2026 |
| Original Interiors (Fibrous Plastering) Limited | Manufacturing | 2004 |
| Orion Mechanical Services (UK) Ltd | Construction | 2009 |
| Orionis Systems Ltd | Health & social care | 2014 |
| Orrells Properties Ltd | Real estate | 2024 |
| Orsen Limited | Construction | 2014 |
| Osborne Accounting & Bookkeeping Ltd | Professional & scientific | 2025 |
| Osborne Fitness & Wellbeing Ltd | Other services | 2026 |
| Osborne's Seafood Services Limited | Financial services | 2025 |
| Oscar and Friends Ltd | Information & communication | 2024 |
| Oscar Maxwell Limited | Hospitality | 2021 |
| Osko Bros Ltd | Real estate | 2023 |
| Osko Bros Property Ltd | Real estate | 2025 |
| OT Therapy Limited | Health & social care | 2023 |
| Otaku DE Japanese Ltd | Professional & scientific | 2009 |
| Other Kids Ltd | Wholesale & retail | 2017 |
| Otherletters Limited | Wholesale & retail | 2020 |
| Otoole Estates Limited | Real estate | 2025 |
| OTT Express Carriers Ltd | Transport & logistics | 2010 |
| OUR Havens Ltd | Health & social care | 2024 |
| OUT of Time Film Ltd | Information & communication | 2023 |
| Outen Tyres Limited | Wholesale & retail | 2012 |
| Outlook Building Services Limited | Construction | 2016 |
| Outward Common Ltd | Financial services | 2025 |
| Ovdigital Limited | Information & communication | 2026 |
| Overall Results Coaching Limited | Arts & recreation | 2025 |
| OWL Valley Enterprises Limited | Professional & scientific | 2012 |
| Own Your Mark Ltd | Wholesale & retail | 2025 |
| Oxley Services & Sons Limited | Construction | 2015 |
| Oxygen Therapy Technical Services Limited | Manufacturing | 2020 |
| Ozrail Limited | Transport & logistics | 2008 |
| Ozturk M S Ltd | Health & social care | 2013 |
| Ozzi Boxes Ltd | Manufacturing | 2022 |
| P & D Material Recovery Ltd | Water & waste | 2016 |
| P & G Miller Developments Ltd | Construction | 2004 |
| P & H Motorcycles Limited | Wholesale & retail | 1994 |
| P Birch Services Ltd | Construction | 2025 |
| P Hawkes Consultancy Ltd | Financial services | 2026 |
| P J I Autos Ltd | Wholesale & retail | 1998 |
| P Stearman CON Ltd | Professional & scientific | 2023 |
| P&P Building Services (Essex) Limited | Construction | 2022 |
| P&P Damp and Timber Services Limited | Construction | 2021 |
| P.Cunningham. Painter, Decorator and Property Maintenance Ltd | Construction | 2007 |
| P.D.T. Worktops Limited | Wholesale & retail | 2013 |
| P.H CONSTRUCTION83 Limited | Construction | 2024 |
| P.Rundell Consulting Limited | Professional & scientific | 2012 |
| PA Associates Limited | Professional & scientific | 2015 |
| Pacific Bathrooms Limited | Construction | 2006 |
| Pacific Installations Limited | Construction | 2010 |
| Packlr Limited | Administrative & support | 2017 |
| Padel (NR) Ltd | Arts & recreation | 2025 |
| Padel LOS Ltd | Extraterritorial | 2024 |
| Paer Consultancy Limited | Information & communication | 2022 |
| Page 32 Fine Wines & Spirits Limited | Wholesale & retail | 2024 |