| Osmani Estates Limited | Real estate | 2024 |
| OSO Property Ltd | Construction | 2019 |
| Ospanova Ltd | Wholesale & retail | 2023 |
| Osprey Elite Organisations (OEO) Ltd | Other services | 2017 |
| OSS Transport Ltd | Transport & logistics | 2017 |
| Ossa Health Care Ltd | Health & social care | 2023 |
| Osteopoise Healthcare Limited | Wholesale & retail | 2002 |
| Ostin Property Management Limited | Financial services | 2016 |
| Ostrich Burger Co Ltd | Hospitality | 2022 |
| Osul Consulting Limited | Extraterritorial | 2019 |
| Oswin's Retreat Ltd | Real estate | 2023 |
| Osza Piekar Ltd | Wholesale & retail | 2017 |
| OT Focus Ltd | Health & social care | 2022 |
| OT Fundraisers CIC | Wholesale & retail | 2022 |
| OT Security Ltd | Administrative & support | 2018 |
| OTA Ltd | Professional & scientific | 2016 |
| OTA Rentals Limited | Real estate | 2024 |
| Otakuri Ltd | Wholesale & retail | 2024 |
| OTB Group Ltd | Financial services | 2024 |
| OTB Legal Limited | Professional & scientific | 2018 |
| Otb-Midas Limited | Wholesale & retail | 2025 |
| Other Heating Solution Ltd | Wholesale & retail | 2022 |
| Other Side Studio Ltd | Professional & scientific | 2021 |
| Othy Cuts Ltd | Other services | 2026 |
| Otisof Limited | Manufacturing | 2022 |
| Otium Legal Limited | Professional & scientific | 2015 |
| Otium Parks Limited | Real estate | 2016 |
| Oto-Tech Professional Audiological Supplies Limited | Wholesale & retail | 2023 |
| Otohns Limited | Health & social care | 2024 |
| Otok Studio Ltd | Professional & scientific | 2010 |
| Otoka Ltd | Arts & recreation | 2022 |
| OTS Properties Limited | Real estate | 2024 |
| OTT Charlbury Ltd | Real estate | 2025 |
| OTT Media Limited | Information & communication | 2023 |
| Otters Tech Limited | Wholesale & retail | 2025 |
| Otto Homes Air CON Ltd | Construction | 2026 |
| Otto Homes Ltd | Real estate | 2020 |
| Otto Media Ltd | Arts & recreation | 2024 |
| OUR Enterprise (Bordon) Ltd | Construction | 2016 |
| OUR Forefathers C.I.C. | Administrative & support | 2017 |
| OUR Hearts Ltd | Professional & scientific | 2024 |
| OUR House Tattoo and Piercing Ltd | Other services | 2025 |
| OUR Lady of Lourdes Catholic Multi-Academy Trust | Education | 2011 |
| OUR Secret Garden Ltd | Hospitality | 2020 |
| OUR World TOO Limited | Information & communication | 2020 |
| Ourami Limited | Professional & scientific | 2020 |
| Ourbnb Ltd | Other services | 2024 |
| Ourhometoyours Limited | Wholesale & retail | 2019 |
| OUT of the HAT Limited | Information & communication | 2000 |
| Outback Interiors Limited | Manufacturing | 2012 |
| Outdoor Scene Limited | Extraterritorial | 1977 |
| Outer Spaces Landscapes Limited | Administrative & support | 2009 |
| Outkruit CIC | Administrative & support | 2023 |
| Outlander Custom Limited | Professional & scientific | 2023 |
| Outlaw Apps Ltd | Information & communication | 2022 |
| Outlaw IT Consultants Ltd | Information & communication | 2026 |
| Outlaw Scaffolding Ltd | Construction | 2024 |
| Outlaws Pickleball Ltd | Education | 2024 |
| Outpostx Ltd | Professional & scientific | 2024 |
| Outre Flicker Ltd | Information & communication | 2026 |
| Outreach Community Group CIC | Education | 2025 |
| Outrider Ltd | Professional & scientific | 2026 |
| Outrun Logistics Ltd | Transport & logistics | 2026 |
| OUTSID3R Ltd | Wholesale & retail | 2024 |
| Outsmart Wind Professionals Limited | Professional & scientific | 2013 |
| Outsource Experts Ltd | Information & communication | 2009 |
| Outsource Supplies Limited | Wholesale & retail | 1998 |
| Outsourced Accounts Office Limited | Professional & scientific | 2025 |
| Outstanding Prospects Limited | Professional & scientific | 2011 |
| Outtabounds Indoor Golf Ltd | Arts & recreation | 2023 |
| Ouuu Ltd | Wholesale & retail | 2026 |
| OV25 Ltd | Information & communication | 2025 |
| OV25 Online Limited | Information & communication | 2026 |
| Oval Cloud Limited | Information & communication | 2011 |
| Ovaltech Limited | Information & communication | 1987 |
| Over Square LEG Limited | Professional & scientific | 1999 |
| Over the Rainbow Nurseries Limited | Education | 2014 |
| Overbeck Property Maintenance Ltd | Construction | 2023 |
| Overdale (Long Eaton) Management Company Limited | Households | 1982 |
| Overdale and Elmhurst Management Company Limited | Households | 2003 |
| Overflow Solution Limited | Health & social care | 2019 |
| Overgrown Audio Ltd | Professional & scientific | 2025 |
| Overlane Properties Ltd | Real estate | 2012 |
| Overline Sports Revenue Strategy Ltd | Professional & scientific | 2023 |
| Overlook Management Group Ltd | Financial services | 2025 |
| Overna Limited | Professional & scientific | 2014 |
| Overnightly Ltd | Hospitality | 2022 |
| Oversolve Limited | Manufacturing | 1992 |
| Overton Site Services Limited | Construction | 2006 |
| Ovexlabs Ltd | Information & communication | 2024 |
| Ovikogo Ltd | Real estate | 2021 |
| Ovproperties Ltd | Real estate | 2018 |
| Owen & Owen Creative Limited | Extraterritorial | 2013 |
| Owen Open Water Education Network Ltd | Education | 2018 |
| Owens Couriers Ltd | Transport & logistics | 2013 |
| OWL EYE Facilities Management Limited | Professional & scientific | 2021 |
| OWL Property Solutions Ltd | Real estate | 2015 |
| OWL Street Limited | Administrative & support | 2020 |
| Own Interior Design Limited | Professional & scientific | 2015 |
| Owss Plus Limited | Construction | 2017 |