| Oasis (Kimberworth) Management Company Limited | Households | 2007 |
| Obsession Giurgiu Ltd | Construction | 2023 |
| Obsidian Stone Limited | Manufacturing | 2010 |
| OC8888 Ltd | Hospitality | 2022 |
| Ocal Solutions Ltd | Professional & scientific | 2026 |
| Ocean Fish Bar PVT Ltd | Hospitality | 2015 |
| Oceane Services Limited | Hospitality | 2025 |
| OCG Homes Limited | Construction | 2009 |
| Octane Interactive Limited | Information & communication | 1999 |
| Octillion Retailers Ltd | Hospitality | 2024 |
| Octoinkjet Limited | Manufacturing | 2011 |
| ODJ Drywall Ltd | Construction | 2023 |
| Odolls Properties Limited | Real estate | 2025 |
| ODP Express Ltd | Public administration | 2023 |
| Oewa UK Limited | Professional & scientific | 2023 |
| OFF Grid Vans Ltd | Extraterritorial | 2021 |
| OFF Road Consultancy Limited | Administrative & support | 2021 |
| Office Cleaning Angels Ltd. | Wholesale & retail | 2014 |
| Office Supplies for Industry Limited | Wholesale & retail | 2000 |
| Ofmr Limited | Other services | 2018 |
| Ogden Lettings Ltd | Real estate | 2025 |
| Ogden Ltd | Professional & scientific | 2026 |
| Ogdens Excavations Limited | Construction | 2016 |
| OH Holdings Ltd | Financial services | 2024 |
| OH YES IT IS Ltd | Arts & recreation | 2021 |
| Ohana for the Community CIC | Health & social care | 2022 |
| OHW Enterprises Limited | Hospitality | 2024 |
| OJD Property Investments Limited | Real estate | 2024 |
| Ojfw Investments Limited | Real estate | 2024 |
| OJL Holdings Limited | Professional & scientific | 2020 |
| OJL Properties Limited | Real estate | 2020 |
| OK Fleet Disposals Limited | Wholesale & retail | 2013 |
| OKU Beauty Limited | Other services | 2022 |
| Olapa Ltd | Real estate | 2016 |
| Old Father Time Limited | Wholesale & retail | 2025 |
| Old George @ FOX Valley Ltd | Hospitality | 2024 |
| Old George Coffee House Properties Limited | Financial services | 2025 |
| Old Mill Property Developments Limited | Real estate | 2015 |
| Old Park Court (Leeds) Limited | Real estate | 2003 |
| Old Retford Court Management Company Ltd | Households | 1997 |
| Old School House Yorkshire Ltd | Financial services | 2016 |
| Old Taylor's (Tavern) Barnsley Limited | Hospitality | 2025 |
| Old Town Barnsley Limited | Financial services | 2013 |
| Olde American Trailer Shop Ltd | Manufacturing | 2016 |
| Oldfields of Barnsley Limited | Wholesale & retail | 1978 |
| Olgus Holdings Limited | Financial services | 2024 |
| OLH Associates Limited | Professional & scientific | 2023 |
| Olive Luxe Co Ltd | Wholesale & retail | 2023 |
| Olive Rose Holiday Hideaways Limited | Hospitality | 2017 |
| Olive Tree Capital Investments Ltd | Real estate | 2026 |
| Oliver Bros Property Services Ltd | Construction | 2025 |
| Oliver J Roberts Limited | Arts & recreation | 2022 |
| Oliver Squared Limited | Real estate | 2024 |
| Olivers (Barnsley) Limited | Administrative & support | 2017 |
| Olivers Childrenswear Limited | Wholesale & retail | 2012 |
| Oliver’S Taxis Limited | Transport & logistics | 2025 |
| OLYMP-MOBILE2 Ltd | Construction | 2023 |
| Omec Engineering Limited | Manufacturing | 1975 |
| Omegamotive Ltd | Information & communication | 2025 |
| Omni Energy Services Limited | Construction | 2025 |
| Omnicare Services Limited | Health & social care | 2017 |
| Omnirevo Ltd | Wholesale & retail | 2026 |
| Omoh DEJ African Store Ltd | Wholesale & retail | 2025 |
| Omtb Limited | Hospitality | 2023 |
| ON Foot Site Management Ltd | Construction | 2006 |
| ON PAR Health Ltd | Wholesale & retail | 2020 |
| ON PAR Nutrition Bar Ltd | Manufacturing | 2021 |
| ON the Run Productions Limited | Construction | 2021 |
| One Degree Finance Limited | Financial services | 2015 |
| One Degree Ltd | Extraterritorial | 2011 |
| One Degree Music Limited | Information & communication | 2018 |
| One OFF Engineering Services Ltd | Construction | 2003 |
| One Over the Eight Ltd | Arts & recreation | 2018 |
| One Plus 3 Holdings Limited | Real estate | 2025 |
| One Plus 3 Limited | Construction | 2022 |
| One Resource Limited | Administrative & support | 2013 |
| One Stop Game Shop Limited | Wholesale & retail | 2011 |
| Onebeyond Retail Limited | Wholesale & retail | 2018 |
| Oneheat Limited | Construction | 2018 |
| Onions Studio Ltd | Professional & scientific | 2013 |
| Online Economics & Business Tuition Limited | Education | 2020 |
| Online Painted Parts Limited | Wholesale & retail | 2022 |
| Only Roofs Limited | Construction | 2009 |
| Onpar Health Community CIC | Other services | 2025 |
| Onside Financial Management Ltd | Professional & scientific | 2013 |
| Onsigns Limited | Arts & recreation | 2017 |
| Onsite Automotive Engineering Ltd | Wholesale & retail | 2026 |
| Onsite Compliance Limited | Construction | 2019 |
| Onyx Conversions Ltd | Wholesale & retail | 2015 |
| Opal Rose Homes Ltd | Real estate | 2025 |
| Open Door Studio Limited | Information & communication | 2024 |
| Open Homes Ltd | Real estate | 2024 |
| Open Network Management Limited | Information & communication | 1994 |
| Open Transport Logistics Ltd | Transport & logistics | 2019 |
| Opening Doors Greenacre C.I.C. | Education | 2014 |
| Operam Education Group Limited | Financial services | 2017 |
| Operam Education Leadership Limited | Other services | 2022 |
| Operam Education Leadership Recruitment Limited | Professional & scientific | 2022 |
| Operam Education Limited | Financial services | 2017 |
| Operam Education Recruitment Limited | Administrative & support | 2023 |