| Operose Store Ltd | Manufacturing | 2025 |
| Opex Groupuk Ltd | Professional & scientific | 2025 |
| Opex Services Ltd | Professional & scientific | 2024 |
| OPH (Projects) Ltd | Construction | 2025 |
| OPM (NW) Ltd | Construction | 2018 |
| Opposite of Chaos Ltd | Professional & scientific | 2017 |
| OPR Investments Ltd | Real estate | 2024 |
| OPS Optimal Plumbing Services Ltd | Construction | 2024 |
| Opsdeck Ltd | Information & communication | 2026 |
| Opsis Consulting (Yorkshire) Limited | Professional & scientific | 2013 |
| Opssynta Limited | Information & communication | 2026 |
| OPT201 Limited | Financial services | 2020 |
| Opteran Technologies Limited | Information & communication | 2020 |
| Opti Haulage Ltd | Transport & logistics | 2024 |
| Optic Circle Limited | Information & communication | 2011 |
| Optidata Communications Ltd | Information & communication | 2018 |
| Optiflow Networks Limited | Professional & scientific | 2023 |
| Optima Asset Management Limited | Administrative & support | 2013 |
| Optima Health (Birmingham) Limited | Health & social care | 2002 |
| Optima Health Group Limited | Administrative & support | 2015 |
| Optima Health Medical Assessments Limited | Health & social care | 2013 |
| Optima Health PLC | Administrative & support | 2022 |
| Optima Health UK Limited | Health & social care | 2013 |
| Optima Safety & Health Ltd | Administrative & support | 2013 |
| Optimal Dent Ltd | Health & social care | 2022 |
| Optimal Electrical Services Ltd | Construction | 2015 |
| Optimo M&E Ltd | Professional & scientific | 2019 |
| Optimum AV Ltd | Administrative & support | 2020 |
| Optimum Business Protection Limited | Financial services | 2024 |
| Optimum Care and Support Ltd | Health & social care | 2023 |
| Optimum Commercial Solutions Ltd | Financial services | 2015 |
| Optimum Elite Ltd | Financial services | 2020 |
| Optimum Estates Limited | Real estate | 2016 |
| Optimum FS Limited | Financial services | 2013 |
| Optimum Harbor Limited | Information & communication | 2025 |
| Optimum Health Solutions (Consultancy) Ltd | Real estate | 2017 |
| Optimum Healthcare Limited | Health & social care | 2015 |
| Optimum Holdings Limited | Financial services | 2019 |
| Optimum Residential Solutions Limited | Financial services | 2025 |
| Optimum RX Limited | Wholesale & retail | 2021 |
| Optimum Sales Results Belper Limited | Administrative & support | 2003 |
| Optimum Sales Results Limited | Professional & scientific | 2003 |
| Optimum Wills & Estate Planning Limited | Professional & scientific | 2024 |
| Optimus Estates Limited | Real estate | 2017 |
| Optimus Security Ltd. | Administrative & support | 2024 |
| Option 1 Property Solutions Ltd | Real estate | 2018 |
| Options Bliss Limited | Other services | 2005 |
| Optrical Limited | Manufacturing | 2016 |
| Opulent Cruise Ltd | Wholesale & retail | 2024 |
| Opulent Dental Services Limited | Health & social care | 2018 |
| Opulon Holdings Ltd | Professional & scientific | 2025 |
| Opus Accountants Ltd | Professional & scientific | 2024 |
| Opus Employee Voice Consulting Ltd | Professional & scientific | 2025 |
| Opus Independents Limited | Information & communication | 2008 |
| Opwg Ltd | Wholesale & retail | 2015 |
| OR Teaching Ltd | Financial services | 2017 |
| ORA DBA Services Ltd | Information & communication | 2014 |
| Ora-Cure Limited | Health & social care | 2022 |
| Oracle Healthcare Ltd | Health & social care | 2018 |
| Oraculi Properties Ltd | Real estate | 2020 |
| Oracy Ltd | Information & communication | 1999 |
| Oral Surgery Education Ltd | Education | 2024 |
| Orange Bird Ltd | Hospitality | 2019 |
| Orange Claw Limited | Information & communication | 2007 |
| Orange Crab Consulting Ltd | Information & communication | 2025 |
| Orange Elite Engineering Limited | Construction | 2024 |
| Orange Stone Properties Ltd | Construction | 2019 |
| Orange to Blue Limited | Hospitality | 2017 |
| Orange Tree Associates Community Interest Company | Wholesale & retail | 2011 |
| Orapiu Limited | Wholesale & retail | 2014 |
| Orbit Media Ltd | Other services | 2015 |
| Orbixa Ltd | Information & communication | 2025 |
| Orbiztech Ltd | Information & communication | 2015 |
| Orchard Court Management (Burncross) Limited | Other services | 2016 |
| Orchard FOX Property Investments Ltd | Real estate | 2025 |
| Orchard Square Social Ltd | Hospitality | 2026 |
| Orchestral Limited | Arts & recreation | 2014 |
| Orchid Building Services Limited | Professional & scientific | 2006 |
| Orchid Orthopedic Solutions Ltd | Professional & scientific | 2012 |
| Orchid Orthopedic Solutions Sheffield Ltd | Manufacturing | 1973 |
| ORD Consulting Ltd | Professional & scientific | 2024 |
| ORD Rail Services Ltd | Professional & scientific | 2025 |
| Orda Gear Ltd | Manufacturing | 2024 |
| Order Order! Limited | Extraterritorial | 2013 |
| Orderra Ltd | Information & communication | 2025 |
| Ordershubs Ltd | Wholesale & retail | 2025 |
| Ordinantia Ltd | Information & communication | 2012 |
| Oren Property Ltd | Real estate | 2023 |
| Organic Partner Ltd | Hospitality | 2024 |
| Organic Touch by Clive Foot Limited | Extraterritorial | 2009 |
| Organisation and Social Development Consultants (Osdc) Limited | Education | 1991 |
| Organised Chaos Event Ltd | Other services | 2026 |
| Organised Chaos Tech Limited | Information & communication | 2024 |
| Orgsoft Ltd | Information & communication | 2016 |
| Oriel Art Advisory Ltd | Wholesale & retail | 2024 |
| Oriel Mount Management Limited | Households | 1971 |
| Orient by Di’Nma Ltd. | Wholesale & retail | 2023 |
| Orient Trans Limited | Transport & logistics | 2018 |
| Oriental RUG Cleaning & Repairs Ltd | Other services | 2024 |
| Oriental RUG Shop Limited | Wholesale & retail | 2010 |