| Objective Asset Group Ltd | Financial services | 2021 |
| Objective Asset Management (Group) Ltd | Real estate | 2021 |
| Objective Computing Limited | Information & communication | 1988 |
| Oblivion Allstars Tumble Limited | Arts & recreation | 2015 |
| OBM Properties Limited | Real estate | 2006 |
| OBN Associates Limited | Real estate | 2010 |
| Obsidian Media Limited | Professional & scientific | 2023 |
| OC Energy Holdings Ltd | Financial services | 2024 |
| OC Energy Ltd | Financial services | 2023 |
| OCC Construct Ltd | Construction | 2023 |
| OCC Finance Limited | Financial services | 2016 |
| Occasions Catering Hire Limited | Hospitality | 1992 |
| Ocean Red Partners Limited | Information & communication | 2023 |
| Oceanbridge Shipping Limited | Professional & scientific | 1995 |
| Oceanform Limited | Wholesale & retail | 2023 |
| Oceania Foods Ltd | Hospitality | 2019 |
| Ockendon Road Limited | Real estate | 2010 |
| OCP Group Limited | Professional & scientific | 2025 |
| Octagon Health Limited | Health & social care | 2011 |
| Octo Property Group Ltd | Real estate | 2025 |
| October Homes Limited | Construction | 2004 |
| October Maple Group Ltd | Manufacturing | 2024 |
| Octofelix Limited | Other services | 2018 |
| Oculus Wealth Management (Chelmsford) Holdings Limited | Financial services | 2025 |
| Oculus Wealth Management (Chelmsford) Limited | Financial services | 2013 |
| Odedra Estates Limited | Real estate | 2019 |
| Odedra Property Holdings Limited | Real estate | 2020 |
| Odhams Air Services Limited | Administrative & support | 2015 |
| Oduenyi Limited | Health & social care | 2019 |
| Odyssey Payroll Limited | Administrative & support | 2022 |
| OFF Road Kids Limited | Administrative & support | 2018 |
| OFF the Farm Tree Ltd | Hospitality | 2019 |
| OFF the Wall Recruitment Limited | Administrative & support | 2017 |
| Offshore Operations Limited | Administrative & support | 2021 |
| Offspring Assets Ltd | Wholesale & retail | 2022 |
| OGW 203 Ltd | Financial services | 2025 |
| OH Forno Limited | Hospitality | 2026 |
| OH Solutions Essex Ltd | Manufacturing | 2025 |
| Ohuk Events Ltd | Administrative & support | 2024 |
| OII Limited | Manufacturing | 2011 |
| OIO Digital Ltd | Information & communication | 2019 |
| Oioi KOI Limited | Wholesale & retail | 2020 |
| OJF Electrical Limited | Construction | 2022 |
| OJF Renewables Limited | Construction | 2025 |
| Okostal Construction Limited | Construction | 2016 |
| Olas Content Ltd | Information & communication | 2021 |
| Old Oak Common Group Holdings Limited | Financial services | 2022 |
| Old Oak Common Holdings Ltd | Real estate | 2021 |
| Old Oak Farm Properties Limited | Construction | 2022 |
| Old Rectory Court Management Company Limited | Households | 1981 |
| Old School House (Chelmsford) Limited | Construction | 2020 |
| Old School House (Moulsham Flats) Ltd | Households | 2026 |
| Oldbury Farm (Great Wakering) Ltd | Agriculture & fishing | 2016 |
| Oldeani Limited | Wholesale & retail | 1987 |
| Olden Demon Limited | Information & communication | 2023 |
| Oldroyd Consulting Ltd | Other services | 2025 |
| OLE Construction Limited | Construction | 2014 |
| Olea Community CIC | Health & social care | 2026 |
| Olex Management Group Ltd | Professional & scientific | 2023 |
| Olidav Ltd | Other services | 2022 |
| Oligen Bookkeeping Services Ltd | Professional & scientific | 2021 |
| Olint Global Services Limited | Health & social care | 2010 |
| Olint Homes Ltd | Real estate | 2020 |
| Olio at the Chequers Limited | Hospitality | 2017 |
| Olio on the Green Ltd | Hospitality | 2017 |
| Olio on the Ridge Ltd | Hospitality | 2012 |
| Olive & Co Holdings Ltd | Hospitality | 2026 |
| Olive Court Properties Limited | Real estate | 2013 |
| Olive Supper Ltd | Hospitality | 2021 |
| Oliver Raine Limited | Administrative & support | 2004 |
| Oliver's Corner Management Company Limited | Households | 2008 |
| Olivers Way (Chelmsford) Management Company Limited | Households | 1985 |
| Olivia and Gray Limited | Wholesale & retail | 2023 |
| Olivia Style Studios Ltd | Other services | 2026 |
| Ollie Harris Media Ltd | Information & communication | 2025 |
| Olov Consultancy Limited | Information & communication | 2011 |
| Olov Estates Limited | Real estate | 2020 |
| Oltco Franchise Holdings Ltd | Financial services | 2026 |
| Oltco Franchise Ltd | Administrative & support | 2018 |
| Olympustechnical Solutions Ltd | Other services | 2023 |
| OM SAI RR Property PVT Ltd | Real estate | 2023 |
| Omalley Property Services Ltd | Real estate | 2023 |
| Ombts Consultancy Limited | Information & communication | 2012 |
| Omega Sustainability Limited | Professional & scientific | 2024 |
| Omega Trans Ltd | Transport & logistics | 2019 |
| Ommez Properties Ltd | Real estate | 2025 |
| Omni Cartel Ltd | Wholesale & retail | 2025 |
| Omnii Health Ltd | Health & social care | 2021 |
| Omop Properties Ltd | Real estate | 2025 |
| Omri 5 Ltd | Real estate | 2018 |
| ON Capital Limited | Financial services | 2024 |
| ON Demand Business Services Ltd | Professional & scientific | 2025 |
| ON the Latch Ltd | Hospitality | 2018 |
| One Desk Solutions Limited | Professional & scientific | 2022 |
| One Love Chelmsford Limited | Hospitality | 2025 |
| One Moto UK Ltd | Wholesale & retail | 2022 |
| One Norton Road RTM Company Limited | Other services | 2004 |
| One One SIX Network Limited | Information & communication | 2020 |
| One SIX QNS Ltd | Wholesale & retail | 2018 |
| One Solution Cloud Limited | Information & communication | 2026 |