| OP Support Limited | Professional & scientific | 2003 |
| Opal GAS Limited | Energy | 2015 |
| Opal Research and Consulting Limited | Professional & scientific | 2005 |
| Opal Utilities Ltd. | Energy | 1996 |
| Opaz Investments (UK) Limited | Real estate | 2024 |
| Open Country | Health & social care | 2004 |
| Open Data Exchange Ltd | Information & communication | 2024 |
| Open Fields Consulting Limited | Professional & scientific | 2009 |
| Open Innovations Ltd. | Information & communication | 2015 |
| Open Space Consulting Ltd | Wholesale & retail | 2022 |
| Opendoor Scientific Ltd | Professional & scientific | 2025 |
| Opening Minds Training and Consultancy Limited | Education | 2016 |
| Openness Healthcare Ltd | Administrative & support | 2025 |
| Operator Experience Ltd | Professional & scientific | 2026 |
| Opex Selection Limited | Administrative & support | 2009 |
| Optechs Networks Limited | Information & communication | 2019 |
| Optima Capital Group Ltd | Financial services | 2023 |
| Optima Group Advisory Ltd | Real estate | 2023 |
| Optimum Health With Patti Hemmings Ltd | Health & social care | 2024 |
| Optimum Performance Arabia Ltd | Professional & scientific | 2023 |
| Optimus Team Learning Limited | Administrative & support | 2016 |
| Opus Yorkshire Ltd | Professional & scientific | 2025 |
| OR1ON Limited | Extraterritorial | 2020 |
| Oracle Constructions Ltd | Construction | 2015 |
| Orangahoo Ltd | Manufacturing | 2017 |
| Orangemaid Consultancy Ltd | Professional & scientific | 2021 |
| Oravaro Ltd | Financial services | 2020 |
| ORB1T Ltd | Extraterritorial | 2026 |
| Orbis Management Limited | Construction | 1996 |
| Orbit FM Limited | Extraterritorial | 2026 |
| Orchard Care Homes.Com (2) Limited | Health & social care | 2007 |
| Orchard Care Homes.Com (6) Limited | Health & social care | 2009 |
| Orchard Care Homes.Com Holdings Limited | Financial services | 2007 |
| Orchard Grove Hunsingore Management Limited | Real estate | 2022 |
| Orchard Project Management Ltd | Arts & recreation | 2015 |
| Orchard Residential Care Limited | Health & social care | 2017 |
| Ordale Agent Ltd | Administrative & support | 2023 |
| Ordantus Ltd | Professional & scientific | 2025 |
| Ordered Space Ltd | Other services | 2021 |
| Orenda N Ltd | Wholesale & retail | 2023 |
| ORGAN1C Limited | Wholesale & retail | 2021 |
| Organic Planet Limited | Extraterritorial | 2011 |
| Orientation Consultancy Services Limited | Information & communication | 1998 |
| Origin Private Office Ltd | Real estate | 2025 |
| Original Marketing Limited | Extraterritorial | 2012 |
| Orium Fire Consultants Ltd | Professional & scientific | 2022 |
| ORL Finance Ltd | Financial services | 2022 |
| Ormond Scott Limited | Administrative & support | 2013 |
| Ornamenti Limited | Extraterritorial | 2021 |
| Orsis Limited | Information & communication | 2006 |
| Orthodox Limited | Extraterritorial | 2012 |
| Orvi International Group Ltd | Wholesale & retail | 2024 |
| OS4D Ltd | Information & communication | 2019 |
| Osbourne Group Limited | Extraterritorial | 2025 |
| Osbourne Logistics Limited | Extraterritorial | 2025 |
| Oscar & Rose Ltd | Professional & scientific | 2022 |
| Oscar 41 Barber Ltd | Other services | 2019 |
| Oscar Martell Limited | Extraterritorial | 2026 |
| Oscorp UK Limited | Real estate | 2012 |
| OSJ Limited | Information & communication | 2010 |
| Osmosis Online Ltd | Administrative & support | 2017 |
| Ostle & Maillard Limited | Information & communication | 2003 |
| OTC Vapes Limited | Wholesale & retail | 2025 |
| Other Productions Limited | Information & communication | 2020 |
| Otium Properties Limited | Real estate | 2019 |
| Otley Road Dental Practice Ltd | Health & social care | 2009 |
| Otoro Labs Ltd | Professional & scientific | 2024 |
| OTT TV Ltd | Information & communication | 2018 |
| Otterington Farm Limited | Financial services | 2024 |
| Ottie and Iris Ltd | Other services | 2023 |
| Otto Cafe Ltd | Hospitality | 2025 |
| Ouseburn Land & Forestry Limited | Real estate | 2024 |
| OUT of Office Club Limited | Other services | 2023 |
| Outdo Media Investments Ltd | Financial services | 2025 |
| Outdoorhobbies Limited | Wholesale & retail | 2012 |
| Outside Inn Harrogate Limited | Hospitality | 2024 |
| Outsourced Team Solutions Limited | Professional & scientific | 2022 |
| Over & Above Bars Ltd | Hospitality | 2025 |
| Overblow Lighting Ltd | Manufacturing | 2021 |
| Overblow Property Ltd | Real estate | 2020 |
| Overherd Brands Ltd | Wholesale & retail | 2022 |
| Owaisco Trading Ltd | Wholesale & retail | 2026 |
| OWL Communications Limited | Information & communication | 2020 |
| OWL Recruitment Yorkshire Ltd | Administrative & support | 2019 |
| Oxview Properties Ltd | Real estate | 2022 |
| OYD GAS Services Ltd | Education | 2023 |
| Oyster Logistics UK Ltd | Other services | 2020 |
| Ozone Global Ltd | Wholesale & retail | 2021 |
| P & J Properties (Yorkshire) Limited | Real estate | 2003 |
| P & K Store Ltd | Wholesale & retail | 2025 |
| P & R Investments Limited | Professional & scientific | 2023 |
| P Gaines Investments Limited | Financial services | 2016 |
| P R Motors (Harrogate) Limited | Wholesale & retail | 2006 |
| P W Installations Yorkshire Limited | Information & communication | 2010 |
| P. R. Records Limited | Information & communication | 1979 |
| P.C Manufacturing Ltd | Manufacturing | 2018 |
| P.Dyke Decorators Limited | Construction | 2003 |
| P.J. Packaging Limited | Manufacturing | 1992 |
| P.M.Management Consultants Limited | Professional & scientific | 1967 |
| P.S. Mill Development LLP | | 2008 |