| Opticalglasses Limited | Wholesale & retail | 2025 |
| Opticals (AZ) Ltd | Wholesale & retail | 2023 |
| Opticians Direct Ltd | Health & social care | 2014 |
| Opticova Limited | Wholesale & retail | 2025 |
| Optident Healthcare Ltd | Health & social care | 2022 |
| Optima Construction & Software Consultants Limited | Information & communication | 2025 |
| Optima Reports Ltd | Health & social care | 2022 |
| Optimal Healthcare Services Group Limited | Health & social care | 2019 |
| Optimal Hybrid Academy CIC | Education | 2026 |
| Optimal Marketing Ltd | Wholesale & retail | 2017 |
| Optimal Performance Centre Ltd | Arts & recreation | 2025 |
| Optimal Recovery Room Ltd | Health & social care | 2025 |
| Optimanage Solutions Limited | Information & communication | 2024 |
| Optiminds Brain Training Ltd | Education | 2014 |
| Optimise AV Limited | Wholesale & retail | 2021 |
| Optimise Talent Ltd | Administrative & support | 2023 |
| Optimised Staffing Solutions Ltd | Administrative & support | 2023 |
| Optimum Business Solutions (NW) Limited | Extraterritorial | 2003 |
| Optimum Carparts Ltd | Wholesale & retail | 2017 |
| Optimus Building Services Limited | Construction | 2013 |
| Optimus Sports Management Limited | Arts & recreation | 2023 |
| Options Autism (3) Limited | Health & social care | 2009 |
| Options Autism (7) Limited | Education | 1999 |
| Options Autism Holdings Limited | Education | 2008 |
| Options Autism Services Limited | Education | 2013 |
| Options Group Holdings Limited | Professional & scientific | 2009 |
| Optipro Consulting Limited | Professional & scientific | 2024 |
| Optix Property & Investments Ltd | Real estate | 2021 |
| Optixa Ltd | Professional & scientific | 2025 |
| Opto Property Developments Limited | Real estate | 2016 |
| Opto Training Limited | Administrative & support | 2007 |
| Optodoc Ltd | Real estate | 2025 |
| Optric Supplies Ltd | Wholesale & retail | 2021 |
| Opulence Brokerage Ltd | Real estate | 2025 |
| Opulenta Ltd | Wholesale & retail | 2024 |
| Opulwood Developments Ltd | Construction | 2019 |
| ORA Cuisine Ltd | Hospitality | 2025 |
| Oracle Capital Management and Energy Ltd | Professional & scientific | 2026 |
| Oracle Construction Projects Ltd | Construction | 2019 |
| Oracle Investments (UK) Limited | Real estate | 2014 |
| Orama Professional Services Limited | Transport & logistics | 2023 |
| Orange Dragon Limited | Wholesale & retail | 2016 |
| Orange Global Ltd | Wholesale & retail | 2025 |
| Orange IT Solutions Ltd | Information & communication | 2021 |
| Orange Tentacle Ltd | Information & communication | 2008 |
| ORB Engineering Limited | Professional & scientific | 2012 |
| ORB7 Limited | Professional & scientific | 2022 |
| Orba Electrical Ltd | Construction | 2021 |
| Orbit Accommodation Ltd | Real estate | 2025 |
| Orbit Intelligence Limited | Professional & scientific | 2015 |
| Orbit Service Station Limited | Transport & logistics | 1990 |
| Orbit Transtech Limited | Transport & logistics | 2013 |
| Orbital Technologies Limited | Information & communication | 2022 |
| Orbley Ltd | Wholesale & retail | 2020 |
| Orbyte Limited | Information & communication | 2025 |
| Orchard Day Nursery (Liverpool) Limited | Education | 2007 |
| Orchard Financial Solutions Ltd | Professional & scientific | 2014 |
| Orchard Gardens Residents Management Company Limited | Households | 2023 |
| Orchard Mart Ltd | Wholesale & retail | 2025 |
| Orderecm Ltd | Wholesale & retail | 2026 |
| Ordinary Glory | Arts & recreation | 2019 |
| Orebtec Ltd | Administrative & support | 2017 |
| Oresa Limited | Administrative & support | 2008 |
| Organic Fusions Ltd | Wholesale & retail | 2019 |
| Oribi Innovations Ltd | Information & communication | 2025 |
| Oriental Properties Ltd | Real estate | 2018 |
| Orif Limited | Health & social care | 2010 |
| Origin IQ Limited | Financial services | 2020 |
| Original Biryaniwala Manchester Ltd | Hospitality | 2025 |
| Original FS Holdings Limited | Financial services | 2004 |
| Original Grand Grill Ltd | Hospitality | 2025 |
| Original Lahori Butt Karahi Ltd | Hospitality | 2024 |
| Original Rebel Limited | Professional & scientific | 2018 |
| Original Steakhouse Co Ltd | Hospitality | 2019 |
| Orignal Desi Point Limited | Hospitality | 2025 |
| Oris UK Limited | Wholesale & retail | 1995 |
| Orla Properties Limited | Real estate | 2016 |
| Orlando Bridge Carpets Limited | Wholesale & retail | 2018 |
| ORM Management Ltd | Professional & scientific | 2019 |
| ORM Ventures Ltd | Financial services | 2025 |
| Ormes 3D CAD Services Ltd | Professional & scientific | 2014 |
| Ormr Limited | Real estate | 2025 |
| Ormskirk (Rent-A-Ford) Limited | Wholesale & retail | 1977 |
| Orobosa Transport and Logistics Limited | Transport & logistics | 2025 |
| Oromia Ltd | Transport & logistics | 2025 |
| Oromiya Cars Ltd | Wholesale & retail | 2017 |
| Orrell Mechanical Limited | Construction | 2019 |
| Orrell Steelwork Limited | Construction | 2019 |
| Orsan Automotive Ltd | Manufacturing | 2025 |
| Orseal Limited | Wholesale & retail | 1961 |
| Orta Investments Ltd | Real estate | 2018 |
| Ortak MAC Ltd | Real estate | 2024 |
| Ortho Nation Ltd | Manufacturing | 2026 |
| Ortopilot Ltd | Information & communication | 2019 |
| Orum Consulting Limited | Administrative & support | 2020 |
| Orum Property Limited | Real estate | 2024 |
| Orwin Construction Ltd | Construction | 2023 |
| OS Solutions Ltd | Information & communication | 2024 |
| Osaaa Ltd | Wholesale & retail | 2022 |
| Oscar Charlie & Co Ltd | Construction | 2015 |