| Oraye Rose Consultants Limited | Construction | 2021 |
| ORB Projects Limited | Real estate | 2022 |
| Orbell Energy Ltd | Professional & scientific | 2025 |
| Orchard Aesthetics Dedham Ltd | Other services | 2025 |
| Orchard Brook Residents Management Co Ltd | Households | 2016 |
| Orchard Brooks Healthcare Limited | Information & communication | 2013 |
| Orchard Cladding Specialists Limited | Construction | 2012 |
| Orchard Construct Limited | Construction | 2007 |
| Orchard Court (Colchester) RTM Company Limited | Households | 2014 |
| Orchard Family Holdings Ltd | Financial services | 2023 |
| Orchard Fruit Holdings Limited | Financial services | 2021 |
| Orchard Gardens (Freeholders) Limited | Households | 2019 |
| Orchard Gardens (Kirby Cross) Management Company Limited | Households | 2017 |
| Orchard Hill Design Limited | Professional & scientific | 2008 |
| Orchard ICT Limited | Information & communication | 2009 |
| Orchard K Consulting Limited | Information & communication | 2008 |
| Orchard K Lettings Limited | Real estate | 2015 |
| Orchard Medical & Psychiatric Services Ltd | Health & social care | 2023 |
| Orchard Photography Limited | Professional & scientific | 2026 |
| Orchard Power Services Limited | Construction | 2019 |
| Orchardworld Holdings Limited | Administrative & support | 2005 |
| Orchardworld Limited | Wholesale & retail | 1989 |
| Orchestrated Social Limited | Information & communication | 2017 |
| Orchid Nails (Whitchurch) Ltd | Other services | 2022 |
| Orchid Star Ltd | Arts & recreation | 2024 |
| Orient Energy Industrial Limited | Energy | 2011 |
| Orient Worldwide (Shipping) Limited | Transport & logistics | 1991 |
| Origin Legal Holdings Limited | Financial services | 2025 |
| Origin Legal Limited | Administrative & support | 2003 |
| Origin Paralegal Limited | Extraterritorial | 2005 |
| Original Business Clothing Ltd. | Wholesale & retail | 2025 |
| Original Reflex Limited | Professional & scientific | 2017 |
| Original Space Limited | Construction | 2001 |
| Orion Engineering Group Ltd | Construction | 2025 |
| Orion Future Technology Limited | Wholesale & retail | 2005 |
| Orion Mgmt Limited | Households | 2026 |
| Orisgbube & Co Ltd | Health & social care | 2018 |
| Orka Consulting Ltd | Real estate | 2005 |
| Ormskirk Investments Limited | Real estate | 2018 |
| Ormskirk Management Limited | Real estate | 2018 |
| Oros Consultancy Ltd | Professional & scientific | 2020 |
| ORP Marshall Ltd | Information & communication | 2016 |
| Orque Ltd | Administrative & support | 2020 |
| Ortho Naqvi Limited | Health & social care | 2022 |
| Orton & Wenlock Limited | Construction | 2011 |
| Orwell Build (Clacton) Limited | Construction | 2021 |
| Orwell Build (Clacton) Management Company Limited | Real estate | 2023 |
| Orwell Consulting Limited | Administrative & support | 2019 |
| Orwell Fitness (Ipswich) Limited | Arts & recreation | 2022 |
| Orwell Fitness (Woodbridge) Ltd | Arts & recreation | 2024 |
| Orwell Fitness Holdings Ltd | Financial services | 2024 |
| Orwell Gate (NO.2) Management Company Limited | Households | 2003 |
| Orwell Insurance Services Limited | Financial services | 1999 |
| Orwell Monitoring Solutions Ltd | Professional & scientific | 2021 |
| Orwell Offshore Ltd | Professional & scientific | 2008 |
| Orwell Offshore Mooring Systems Limited | Manufacturing | 2016 |
| Orwell Projects Limited | Construction | 2025 |
| Orwell Quarters Limited | Hospitality | 2023 |
| Orwell Quay (Ipswich) Management Company Limited | Households | 2003 |
| Orwell Securities (Ipswich) Limited | Financial services | 1978 |
| Orwellbuild Limited | Construction | 2018 |
| Os-Five Limited | Health & social care | 2019 |
| OSA Digital Ltd | Professional & scientific | 2019 |
| Osassy Ltd | Wholesale & retail | 2023 |
| Osbel Media Ltd | Information & communication | 2012 |
| Osborn Design & Build Ltd | Construction | 2015 |
| Osborn London Ltd | Wholesale & retail | 2009 |
| Osborne & Sons (Shellfish) Limited | Manufacturing | 1995 |
| Osborne & Sons Properties Limited | Real estate | 2021 |
| Osborne Bros Limited | Wholesale & retail | 2017 |
| Osborne Manor (Colchester) Management Company Limited | Households | 2017 |
| Osborne Property Consultants Limited | Real estate | 2017 |
| Osdon Limited | Wholesale & retail | 2023 |
| OSF Services Limited | Construction | 2026 |
| OSG Groups LLP | | 2019 |
| OSH Dressage Ltd | Agriculture & fishing | 2018 |
| Oshowme Limited | Arts & recreation | 2025 |
| Osier Plumbing Services Ltd | Construction | 2019 |
| OSR Properties Limited | Real estate | 2023 |
| Osrow Group Limited | Financial services | 2020 |
| Osrow Homes Limited | Construction | 2020 |
| Osteo & Chiro Healthcare Ltd | Health & social care | 2024 |
| Osubare Ltd | Health & social care | 2019 |
| OT Management Ltd | Wholesale & retail | 2019 |
| OT Marketing Ltd | Arts & recreation | 2025 |
| Othello Blinds Limited | Other services | 2003 |
| Otis United Limited | Health & social care | 2025 |
| Otium Training & Response Ltd | Administrative & support | 2017 |
| Otod Management Limited | Arts & recreation | 2018 |
| OTT (UK) Limited | Real estate | 1994 |
| Otter Housing Ltd | Real estate | 2021 |
| Ottking Ltd | Households | 2026 |
| Otto Pizza Ltd | Hospitality | 2024 |
| Ottoman Media & Trading Ltd | Wholesale & retail | 2020 |
| Ouch! Tattoo & Piercings Ltd | Arts & recreation | 2025 |
| OUI Cest BON Ltd | Manufacturing | 2025 |
| Oulton Hall Management Limited | Households | 1982 |
| OUR Colchester Limited | Administrative & support | 2018 |
| OUR Deed Ltd | Real estate | 2025 |
| OUR Investments Ltd | Real estate | 2025 |