| OCG Contracting Ltd | Construction | 2022 |
| Ochello Services Ltd | Professional & scientific | 2023 |
| Ochy Freshclean Service Limited | Administrative & support | 2025 |
| Ocia Ltd | Information & communication | 2015 |
| Ocia Offices Limited | Real estate | 2023 |
| Ocia Spark Ltd | Information & communication | 2022 |
| Ockine Investment UK Limited | Transport & logistics | 2008 |
| OCR Land Ltd. | Construction | 2013 |
| OCR Motivational Ltd | Administrative & support | 2022 |
| Ocrom Resourcing Ltd | Construction | 2024 |
| Octagon Grill & Bar Ltd | Hospitality | 2025 |
| Octavia's Revenge Ltd | Professional & scientific | 2025 |
| Octo Healthcare Limited | Financial services | 2020 |
| Octopus Consulting Limited | Information & communication | 2006 |
| Octopus Gates Automated Access LLP | | 2020 |
| OD Workforce Solutions Limited | Transport & logistics | 2025 |
| Odaci Homes Limited | Construction | 2026 |
| ODD Socks Lettings Ltd | Real estate | 2018 |
| Oddique Media Ltd | Information & communication | 2025 |
| Oddleaf Ltd | Agriculture & fishing | 2025 |
| Odds Farm Park Day Nursery Ltd | Health & social care | 2018 |
| Odell and Company Limited | Wholesale & retail | 1915 |
| Odell Commercial Limited | Extraterritorial | 2024 |
| Odell-Phillips Recruitment Ltd | Professional & scientific | 2020 |
| Odessos Electrical Services Ltd | Construction | 2018 |
| Odin-X Agency Ltd | Professional & scientific | 2023 |
| ODO Ayaresa Care Ltd | Public administration | 2023 |
| ODO Solutions Property Ltd | Real estate | 2025 |
| ODO Solutions Ymdo Consulting Ltd Limited | Construction | 2025 |
| Oduma Solutions UK Limited | Information & communication | 2023 |
| OEL Solutions Ltd | Professional & scientific | 2017 |
| OEM Drive Ltd | Wholesale & retail | 2023 |
| Oenneo Limited | Information & communication | 2019 |
| Oenox Limited | Wholesale & retail | 2021 |
| OER Marketing Ltd | Professional & scientific | 2024 |
| Oerlikon Balzers Coating UK Limited | Manufacturing | 1960 |
| Oerlikon Metco (UK) Limited | Administrative & support | 2003 |
| OFF the Quay Limited | Hospitality | 1992 |
| OFF World Corp Ltd | Wholesale & retail | 2020 |
| Office Green Limited | Wholesale & retail | 1999 |
| Office Stationery Limited | Administrative & support | 2011 |
| Official Injury Claim Limited | Information & communication | 2019 |
| Offset Projects CIC | Education | 2021 |
| Oforo Ltd | Information & communication | 2025 |
| Ogien Testing Ltd | Professional & scientific | 2020 |
| OGN Properties Ltd | Real estate | 2016 |
| Ogochukwuezeocha Limited | Health & social care | 2020 |
| OH Consulting Limited | Professional & scientific | 2026 |
| OH Prime Ltd | Health & social care | 2013 |
| OH SEE Ltd | Professional & scientific | 2018 |
| OH SIX Ltd | Professional & scientific | 2024 |
| Ohana Breaks Limited | Real estate | 2020 |
| Ohana Properties Ltd | Real estate | 2025 |
| Ohealth Company Limited | Information & communication | 2022 |
| Oheneba Childcare Ltd | Health & social care | 2021 |
| OHM Consultancy Limited | Information & communication | 2014 |
| Ohmslaws Ltd | Professional & scientific | 2018 |
| Oicct Ltd | Education | 2013 |
| OIL House Ltd | Wholesale & retail | 2025 |
| Oiserialdeals Limited | Wholesale & retail | 2025 |
| Oishii Sushi Limited | Hospitality | 2019 |
| OJ Build Ltd | Construction | 2015 |
| OJ Chapman Group Ltd | Real estate | 2025 |
| OJ Chapman Holdings Ltd | Financial services | 2025 |
| OJA Capital Ltd | Financial services | 2017 |
| Ojasvi Ltd | Wholesale & retail | 2025 |
| OJC Construction Ltd | Construction | 2019 |
| OJE Consulting Limited | Other services | 2013 |
| OJG Barbering Limited | Other services | 2025 |
| OK Fitness Ltd | Health & social care | 2022 |
| OK Rentals Limited | Real estate | 2014 |
| Okamoda Ltd. | Information & communication | 2010 |
| Okiedoki Ltd | Hospitality | 2025 |
| Okunoin Ltd | Real estate | 2019 |
| OLA Noble Services Limited | Extraterritorial | 2000 |
| OLA OLA Ltd | Information & communication | 2018 |
| Olaluxe Enterprises Ltd | Wholesale & retail | 2025 |
| Olanna Consulting Ltd | Information & communication | 2016 |
| Olcorp Limited | Information & communication | 2013 |
| Old Bath House and Community Centre, Wolverton | Other services | 1993 |
| Old Dirty Brasstards Limited | Arts & recreation | 2013 |
| Old Farm Dogs Limited | Agriculture & fishing | 2025 |
| Old Farmhouse Properties Ltd | Real estate | 2022 |
| Old Gold Records Limited | Professional & scientific | 1996 |
| Old Heath House Estates Ltd | Real estate | 2016 |
| Old Monk Hospitality Ltd | Hospitality | 2026 |
| Old Print Works (Newport Pagnell) Limited | Households | 1997 |
| Old School Rehearsal Rooms Ltd | Arts & recreation | 2024 |
| Old Stratford Decorators Ltd | Construction | 2015 |
| Old Timers Vehicle Hire Ltd | Administrative & support | 2025 |
| Old Town Live Limited | Arts & recreation | 2025 |
| Oldaker Holdings Limited | Construction | 1973 |
| Oldbrook Hire Ltd | Administrative & support | 2013 |
| Oldbrook PJ Pizza Limited | Hospitality | 2019 |
| Oldstone Consultants Limited | Professional & scientific | 2010 |
| Oldupai Development Masters {Cohaa} Ltd | Other services | 2025 |
| OLE UK Production Music Limited | Information & communication | 2012 |
| OLE Wolff Electronics Ltd | Manufacturing | 1998 |
| Olec IMS Ltd | Construction | 2026 |
| Oleevo Ltd | Hospitality | 2018 |