| Olive Plant Hire Ltd | Extraterritorial | 2024 |
| Olive Ranch Holdings Limited | Real estate | 2020 |
| Olive Rose Motor Company Ltd | Wholesale & retail | 2004 |
| Olive Studio Ltd | Arts & recreation | 2024 |
| Oliveos Ltd | Wholesale & retail | 2025 |
| Oliver Tappin Limited | Administrative & support | 2016 |
| Olivia and Oliver Limited | Wholesale & retail | 2020 |
| Olivia Tapas Group Ltd | Hospitality | 2019 |
| Ollivers Trading Limited | Real estate | 2005 |
| Olly Dean Production Ltd | Professional & scientific | 2016 |
| Olma Holdings Limited | Financial services | 2023 |
| OM Provision Limited | Health & social care | 2016 |
| OM SAI RAM Restaurants Limited | Hospitality | 2019 |
| Omega Estates Southern Limited | Real estate | 1999 |
| Omif International Commodities Ltd | Professional & scientific | 2015 |
| Omni (Brokers) Limited | Financial services | 1961 |
| Omni Growth Industries Capital Ltd | Administrative & support | 2018 |
| Omni-Media Ltd | Professional & scientific | 2022 |
| Omraf Limited | Professional & scientific | 2024 |
| Omro Property Limited | Real estate | 2022 |
| ON Course Development Limited | Professional & scientific | 2008 |
| ON Point Rail Services Ltd | Wholesale & retail | 2023 |
| ON the Pulse Consultancy Ltd | Other services | 2017 |
| On-The-One Media Limited | Information & communication | 2008 |
| Onarach Monkey Ltd | Wholesale & retail | 2021 |
| Once Upon Again Limited | Wholesale & retail | 2024 |
| One Account Mobile Ltd | Information & communication | 2018 |
| One Flower Shop Ltd | Wholesale & retail | 2021 |
| One Hyde Park Limited | Households | 2006 |
| One Lane Studios Limited | Wholesale & retail | 2023 |
| One Parking Limited | Extraterritorial | 2012 |
| One Parking Solution Ltd | Administrative & support | 2006 |
| One Path Care Ltd | Health & social care | 2025 |
| One Pulse Ltd | Information & communication | 2026 |
| One Resourcing Holdings Limited | Households | 2018 |
| One Vehicle Logistics Ltd | Transport & logistics | 2016 |
| Onebuild Sussex Limited | Construction | 2023 |
| Oneira Techne Ltd | Professional & scientific | 2018 |
| Onit Limited | Administrative & support | 2017 |
| Online Takeover Music Limited | Information & communication | 2020 |
| Only Natural Products Limited | Wholesale & retail | 1999 |
| Onsite Labour Solutions Ltd | Administrative & support | 2009 |
| Onweb Solutions Limited | Information & communication | 1999 |
| Onyva Travel Ltd | Administrative & support | 2025 |
| Oodles of Stuff Ltd | Wholesale & retail | 2023 |
| OOH LA LA Industries Ltd | Information & communication | 2022 |
| Oosh TCG Ltd | Professional & scientific | 2025 |
| Open Chilli Trading Ltd | Wholesale & retail | 2023 |
| Open Finance Consulting Limited | Administrative & support | 2020 |
| Open Road Property Ltd | Real estate | 2025 |
| Openshaw & Co. Limited | Professional & scientific | 2012 |
| Openspace Developments Sussex Ltd | Construction | 2021 |
| OPL Worthing Limited | Wholesale & retail | 2022 |
| Opportune Management Consultancy Limited | Other services | 2015 |
| Opportunity.Network.Empower Ltd | Health & social care | 2022 |
| Optical (Sussex) Ltd | Health & social care | 2023 |
| Optical GAS Imaging Ltd | Professional & scientific | 2022 |
| Optimal Medical Care Limited | Health & social care | 2005 |
| Optimis Technology Limited | Information & communication | 2020 |
| Optimise Your Software Ltd | Information & communication | 2023 |
| Optimum Travels & Tourism Ltd | Administrative & support | 2022 |
| Option Hygiene South Limited | Wholesale & retail | 2008 |
| Opto Group Holding Limited | Financial services | 2024 |
| Opule Limited | Wholesale & retail | 2025 |
| Oracle Fine Wines Ltd. | Wholesale & retail | 2018 |
| Oracle Glazing & Repairs Ltd | Construction | 2024 |
| Oracle Productions (UK) Limited | Wholesale & retail | 2003 |
| Orange DOT Productions Limited | Information & communication | 2020 |
| Orange Lime Investments Ltd | Professional & scientific | 2025 |
| Orange Smarty Ltd | Information & communication | 2013 |
| ORANGE22 Limited | Manufacturing | 2016 |
| Oratile Kidd Ltd | Professional & scientific | 2024 |
| Orbit Private Holdings I Ltd | Financial services | 2021 |
| Orbit Private Holdings II Ltd | Financial services | 2021 |
| Orca Yacht LLP | | 2025 |
| Orchard Estates (Sussex) Limited | Other services | 2013 |
| ORE Supplies Limited | Administrative & support | 2021 |
| Oreham Group Limited | Professional & scientific | 2019 |
| Oremus Dental Ltd | Health & social care | 2017 |
| Orgtrans Limited | Information & communication | 2014 |
| Origin 81 Designs Ltd | Manufacturing | 2023 |
| Origin Boutique Ltd | Wholesale & retail | 2026 |
| Original Curryland Limited | Hospitality | 2015 |
| Original Interiors Limited | Construction | 2025 |
| Original Monetaire Accountants Limited | Professional & scientific | 2006 |
| Original PUB Holdings LLP | | 2019 |
| Original PUB Investments Limited | Real estate | 2022 |
| Orion Archery Ltd | Arts & recreation | 2020 |
| Orion Dental Ltd | Health & social care | 2016 |
| Orison Properties Limited | Real estate | 2013 |
| Orla James Ltd | Wholesale & retail | 2018 |
| ORM Consulting Limited | Professional & scientific | 2004 |
| Ortho-Services Limited | Health & social care | 2003 |
| Osborne Sales & Marketing Ltd | Professional & scientific | 2013 |
| Oscar Bears Day Care Ltd. | Education | 2011 |
| Oscar Football Club Ltd | Arts & recreation | 2012 |
| Oscar’S Yard Limited | Real estate | 2022 |
| Oscillation Ltd | Professional & scientific | 2012 |
| Osiris Group Ltd | Construction | 2024 |
| OSM Retail Limited | Wholesale & retail | 2020 |