| Nuraghe Group Ltd | Professional & scientific | 2026 |
| Nurai Ltd | Transport & logistics | 2020 |
| Nuts Ritual Ltd | Hospitality | 2025 |
| Nutshell Media Limited | Arts & recreation | 2001 |
| NV Vision Ltd | Health & social care | 2024 |
| Nvgc Ltd | Arts & recreation | 2026 |
| NWG Services Ltd | Administrative & support | 2015 |
| NYG Holdings Ltd | Financial services | 2023 |
| O&O Property Investment Ltd | Real estate | 2026 |
| O&O(Hove) Ltd | Wholesale & retail | 2022 |
| O'Neill's of Brighton Cleaning Services Limited | Administrative & support | 2014 |
| O'Sullivan Ventures Ltd | Financial services | 2024 |
| O-Scr Ltd | Wholesale & retail | 2020 |
| Oades Builders Ltd | Construction | 2010 |
| Oakhouse Associates Limited | Professional & scientific | 1995 |
| Oaklands Kitchens Limited | Construction | 2001 |
| Oakley Autos Ltd | Wholesale & retail | 2020 |
| Oakley Development and Project Management Limited | Construction | 2024 |
| Oakley Property (Shoreham) Limited | Real estate | 2017 |
| Oakley Property (Sussex) Limited | Real estate | 1996 |
| Oakley Residential (Lewes) Limited | Real estate | 2014 |
| Oakmont Financial Advice Limited | Financial services | 2022 |
| Oaks Rise CIC | Education | 2023 |
| Oaks Trust | Health & social care | 2012 |
| Oakwish Limited | Households | 1996 |
| OAS Plumbing & Heating Ltd | Construction | 2026 |
| OB GAS Services Limited | Construction | 2020 |
| Object AD Solutions Limited | Professional & scientific | 2013 |
| Object-Source LLP | | 2008 |
| OBR Ventures Ltd | Administrative & support | 2020 |
| Observant Films Ltd | Information & communication | 2017 |
| OC Fulfillment Solutions Ltd | Construction | 2024 |
| Ocean Breeze Rooms Ltd | Hospitality | 2018 |
| Ocean Executive Security Solutions Ltd | Administrative & support | 2025 |
| Ocean Motors Ltd | Wholesale & retail | 2023 |
| Oceanique Management Company Limited | Real estate | 2006 |
| Oceans Property Management Ltd | Real estate | 2022 |
| Ocelli XP Limited | Professional & scientific | 2019 |
| OCH RTM Company Limited | Households | 2010 |
| Octoppuss Ltd | Manufacturing | 2017 |
| Octopus Direct Limited | Administrative & support | 2019 |
| Octopus Estimating Limited | Energy | 2014 |
| ODD Even Ltd | Hospitality | 2020 |
| ODM (UK) Limited | Administrative & support | 2000 |
| Odon Solutions Limited | Administrative & support | 2026 |
| Odysea Signs Limited | Manufacturing | 2019 |
| Odyssean Enterprises Ltd | Information & communication | 1998 |
| Oeuf Limited | Wholesale & retail | 2020 |
| OFF Course Bookmakers Limited | Arts & recreation | 1986 |
| OFF the Fence Marketing Limited | Professional & scientific | 2013 |
| OFF the Fence Trust Limited | Health & social care | 2004 |
| OFF the Wall Publishing Limited | Information & communication | 2023 |
| Offchain Research Ltd | Information & communication | 2024 |
| Offers on Demand Ltd | Wholesale & retail | 2024 |
| Offerson App Limited | Information & communication | 2025 |
| Office Christmas Ltd | Arts & recreation | 2022 |
| OG and Bones Limited | Real estate | 2024 |
| Oganyx Ltd | Wholesale & retail | 2020 |
| OGB Sussex Limited | Construction | 2025 |
| Ogdens (Brighton) Limited | Real estate | 1939 |
| Ogee Sound Ltd | Manufacturing | 2022 |
| OGS Installations Ltd | Administrative & support | 2023 |
| OH Goods for YOU Limited | Wholesale & retail | 2021 |
| OH SO Sharp Ltd | Other services | 2016 |
| OK DUN Ltd | Information & communication | 2026 |
| OKM Nurseries Ltd | Health & social care | 2022 |
| Old Conna Holdings Limited | Information & communication | 2017 |
| Old Court House Residents Limited | Households | 2004 |
| Old Mill Property Limited | Real estate | 2020 |
| Old School Rewind Ltd | Wholesale & retail | 2024 |
| Old World Trading Company Limited | Wholesale & retail | 2016 |
| Oleka Creative Ltd | Arts & recreation | 2025 |
| OLI Comber Ltd | Information & communication | 2016 |
| OLI With Grace Limited | Health & social care | 2020 |
| Olive & Joyce Limited | Health & social care | 2023 |
| Oliver & Graimes Design Associates Limited | Professional & scientific | 1985 |
| Oliver Moore Fitness Ltd | Arts & recreation | 2020 |
| Oliver's Battery Limited | Real estate | 1994 |
| Olivia Mosley Companies Limited | Professional & scientific | 2016 |
| Ollie Electrical Ltd | Construction | 2024 |
| OLT Holdings Ltd | Financial services | 2020 |
| Olus Biomass Limited | Water & waste | 2012 |
| Olus Environmental Limited | Water & waste | 1983 |
| Olytrad Ltd | Wholesale & retail | 2009 |
| OM Consult Limited | Administrative & support | 2025 |
| Omacast Ltd | Professional & scientific | 2021 |
| Omani Estates Limited | Real estate | 2012 |
| Omani Finance Limited | Financial services | 2012 |
| Omani Holdings Limited | Extraterritorial | 2012 |
| Omani Properties Limited | Real estate | 2012 |
| Omega CS Ltd | Transport & logistics | 2023 |
| Omegaco Property Investments Ltd | Construction | 2019 |
| Omer FC Limited | Administrative & support | 2024 |
| Omnia&Dina Limited | Wholesale & retail | 2015 |
| Omnichain Nexus Ltd | Administrative & support | 2020 |
| ON Brief Ltd | Construction | 2015 |
| ON Budget Limited | Professional & scientific | 2012 |
| ON TAP Property Services Ltd | Construction | 2007 |
| ON the Rocks Hastings Limited | Hospitality | 2019 |
| ON This Land Community Interest Company | Education | 2025 |