| NYM Ltd | Wholesale & retail | 2010 |
| NYM Property Rentals Ltd | Households | 2025 |
| Nymi UK Limited | Information & communication | 2018 |
| Nyos (York) Limited | Health & social care | 2009 |
| Nyxor Group Limited | Financial services | 2026 |
| NZ Backpackers Ltd | Administrative & support | 2018 |
| Nztworx Ltd | Professional & scientific | 2025 |
| O & Z York Limited | Wholesale & retail | 2025 |
| O J Parker Ltd | Professional & scientific | 2024 |
| O Tree York 2 Ltd | Hospitality | 2025 |
| O&Bc Limited | Hospitality | 2021 |
| O&Gp Ltd | Real estate | 2025 |
| O'Beirne Consultant Limited | Information & communication | 2016 |
| O'Neill Planning Limited | Professional & scientific | 2023 |
| O'Neill Portfolio and Investments Limited | Administrative & support | 2015 |
| O24 Films Ltd | Information & communication | 2025 |
| Oak & Stone Construction York Ltd | Construction | 2025 |
| Oak and Slate Limited | Construction | 2018 |
| Oak Apple Decor Limited | Construction | 2010 |
| Oak by Design Holdings Limited | Financial services | 2024 |
| Oak by Design Ltd | Manufacturing | 2002 |
| Oak Cloud Software Limited | Information & communication | 2024 |
| Oak Farming | Financial services | 2009 |
| Oak Farming Renewables Limited | Energy | 2019 |
| Oak Hill Rise (Chippenham) Management Company Limited | Households | 2025 |
| Oak House Civil Engineering Limited | Professional & scientific | 2026 |
| Oak House Veterinary Centre Ltd. | Professional & scientific | 2004 |
| Oak Real Estate Ltd | Professional & scientific | 2020 |
| Oakcroft Chase (Stubbington) Management Company Limited | Households | 2022 |
| Oakhurst 2005 Management Company Limited | Households | 2005 |
| Oakhurst Village (Shirley) Management Company Limited | Households | 2020 |
| Oakland Farms Holdings Limited | Agriculture & fishing | 2025 |
| Oakland Feeds Limited | Professional & scientific | 1981 |
| Oakland Fuels Limited | Agriculture & fishing | 2013 |
| Oakland Properties (Yorkshire) Ltd | Real estate | 2004 |
| Oakland UK Limited | Professional & scientific | 1982 |
| Oakland York Ltd | Construction | 2022 |
| Oaklands Park (Acomb) Management Company Limited | Households | 2002 |
| Oaklea Nurseries Limited | Agriculture & fishing | 1985 |
| Oakleaf Conservatories Limited | Construction | 1986 |
| Oakley Lettings Limited | Real estate | 2013 |
| Oaktree Architectural Limited | Professional & scientific | 2018 |
| Oakville Court Management Company Limited | Households | 2000 |
| Oakwood Barns Owners Management Company Ltd | Households | 2023 |
| Oakwood Caravan Storage Limited | Transport & logistics | 2003 |
| Oakwood Clump Ltd | Hospitality | 2022 |
| Oakwood Landscaping Limited | Administrative & support | 2015 |
| Oakwood Park (Wymondham) Residents Management Company Limited | Households | 2015 |
| Oakwood Resorts Ltd | Hospitality | 2024 |
| Oakwood Veterinary Practice Limited | Professional & scientific | 2017 |
| Oakwood Veterinary Referrals Limited | Extraterritorial | 2015 |
| Oakwood View (Brackla) Management Company Limited | Households | 2017 |
| Oakwood View (Weston-Super-Mare) Management Company Limited | Real estate | 2018 |
| Oakwood Village Lodges Limited | Real estate | 2016 |
| Oasis B&B Ltd | Hospitality | 2025 |
| Oasis of York Limited | Hospitality | 2024 |
| OASIS4LIFE (F&D) Healthcare Ltd | Health & social care | 2020 |
| OAT Residence Limited | Hospitality | 2025 |
| Oates Investments Ltd | Real estate | 2025 |
| OB Automotive Ltd | Wholesale & retail | 2026 |
| Obat Trading Limited | Professional & scientific | 2014 |
| Obispo Group Ltd | Professional & scientific | 2025 |
| Objitec Limited | Information & communication | 2015 |
| OBK Health Ltd | Health & social care | 2022 |
| OBM Vanguard Ltd | Real estate | 2025 |
| Obveris Limited | Information & communication | 2025 |
| OC Properties York Limited | Real estate | 2022 |
| Occaney Close Management Company Limited | Households | 2022 |
| Occupational Awards Limited | Education | 2011 |
| Ocean Corals & Reptiles Ltd | Wholesale & retail | 2009 |
| Ocean Digital Technology (UK) Ltd | Wholesale & retail | 2018 |
| Ocean Lullaby Ltd | Real estate | 2020 |
| Oceans T&M Limited | Hospitality | 2022 |
| OCH (Haxby) Limited | Real estate | 2003 |
| Ochre Red Consulting Ltd | Professional & scientific | 2014 |
| OCK Health Limited | Health & social care | 2006 |
| Ocular Outcomes Ltd | Information & communication | 2018 |
| Oculus (Harrogate) Limited | Wholesale & retail | 1993 |
| Oddenhill Ltd | Information & communication | 2012 |
| Odds on Marketing Limited | Professional & scientific | 2017 |
| Odel Industrial Services Ltd | Wholesale & retail | 2008 |
| Odel Solutions Ltd | Professional & scientific | 2024 |
| Odin Windows Ltd | Construction | 2024 |
| Odyssey Attractions Ltd | Wholesale & retail | 2022 |
| OE Inc Ltd | Wholesale & retail | 2024 |
| OE Media Ltd | Information & communication | 2017 |
| OE Qorban Consult Limited | Information & communication | 2013 |
| Oemsecrets Limited | Professional & scientific | 2005 |
| OES (UK) Limited | Wholesale & retail | 2013 |
| Oezone Ltd | Professional & scientific | 2003 |
| Ofelya Ltd | Other services | 2022 |
| OFF Piste Yorkshire Ltd | Hospitality | 2018 |
| OFF Site Construction Advice Ltd | Professional & scientific | 2012 |
| Offbeat Events & Nightlife Limited | Professional & scientific | 2020 |
| Offer Ready Ltd | Real estate | 2024 |
| Officially Vowd Limited | Other services | 2026 |
| Offshore Companies Limited | Professional & scientific | 2005 |
| Offshore Company Formations Limited | Professional & scientific | 2002 |
| Offshore Surveys International Limited | Professional & scientific | 1974 |
| Offsite Construction Consultants Ltd | Construction | 2020 |