| Old Market Express Ltd | Wholesale & retail | 2024 |
| Old Market Holding Company Limited | Real estate | 2020 |
| Old Market Local Ltd | Wholesale & retail | 2025 |
| Old Market Manor C.I.C. | Real estate | 2014 |
| Old Market Spa Ltd | Other services | 2025 |
| Old Mill Court (Swindon) Management Company Limited | Households | 2003 |
| Old Mills Court Limited | Real estate | 2024 |
| Old Mills Estates Limited | Construction | 1996 |
| Old Mission Management Company | Real estate | 2017 |
| Old Oak Stables Ltd | Other services | 2025 |
| Old Orchard Group Ltd | Construction | 2024 |
| Old Orchard Inns Ltd | Hospitality | 2019 |
| Old Parish Ltd | Real estate | 2023 |
| Old Park Bath Limited | Real estate | 2020 |
| Old PIT Garage Ltd | Wholesale & retail | 2003 |
| Old Restoration GC Ltd | Hospitality | 2026 |
| Old School Court (Bristol) Limited | Households | 2013 |
| Old School Property Maintenance and Management Ltd | Construction | 2019 |
| Old Station Limited | Real estate | 2012 |
| Old Surgery Chew Magna Management Company Limited | Real estate | 2015 |
| Old Town Sounds Ltd | Hospitality | 2013 |
| Old Town Village Limited | Construction | 2019 |
| Old Vicarage Flat Management Limited | Households | 2005 |
| Old Vicarage Green (Keynsham) Management Company Limited | Households | 1990 |
| Old Weston RD Property Management Ltd | Households | 2016 |
| OLD2NEW Windows and Doors Ltd | Construction | 2026 |
| Oldbridge Solar Farm Limited | Energy | 2022 |
| Oldbury Local Ltd | Wholesale & retail | 2021 |
| Oldfashionedbars Ltd | Hospitality | 2018 |
| Oldfield Circus Residents Management Company Limited | Households | 2014 |
| Oldfield Fish & Chips Ltd | Hospitality | 2003 |
| Oldfields Construction Ltd | Other services | 2016 |
| Oldland Common LLP | | 2019 |
| Oldland Contract Management Ltd | Professional & scientific | 2016 |
| Oldland Properties Limited | Real estate | 2024 |
| Oldmead Management Company Limited | Households | 2017 |
| Oldown Tree Surgeons Ltd | Administrative & support | 2018 |
| Olds Group Limited | Real estate | 2019 |
| Olerham LP | | 2013 |
| Olfasense UK Ltd | Professional & scientific | 1994 |
| Olga TV Limited | Information & communication | 2005 |
| OLI Beck Cycling Ltd | Arts & recreation | 2024 |
| OLI Systems, Ltd. | Information & communication | 2019 |
| Olibek Limited | Professional & scientific | 2019 |
| Olikob Ltd | Information & communication | 2023 |
| Olimoto Ltd | Wholesale & retail | 2020 |
| Olimp Interiors Ltd | Construction | 2015 |
| Olira Ltd | Professional & scientific | 2010 |
| Oliva Consultants Ltd | Other services | 2020 |
| Olive and IVY Wellbeing Ltd | Administrative & support | 2023 |
| Olive and Lulu Ltd | Real estate | 2020 |
| Olive and Malbec Properties Ltd | Construction | 2021 |
| Olive Carpentry & Build Ltd | Construction | 2025 |
| Olive Construction Design & Build Limited | Construction | 2010 |
| Olive Construction South West Limited | Construction | 2024 |
| Olive Hire Limited | Transport & logistics | 2025 |
| Olive Interiors Ltd | Construction | 2011 |
| Olive Leaf Homes & Property Sourcing Limited | Real estate | 2021 |
| Olive Leaf Properties Limited | Real estate | 2018 |
| Olive Tree Beauty Therapy Limited | Other services | 2018 |
| Olive Tree Caterers Ltd | Hospitality | 2025 |
| Olive Tree Consulting Ltd | Information & communication | 2018 |
| Oliveira Construction & Maintenance Limited | Construction | 2026 |
| Oliver Cane Video Editor Limited | Information & communication | 2015 |
| Oliver Charles Partnership Limited | Real estate | 2015 |
| Oliver Corbett Interiors Limited | Construction | 2001 |
| Oliver Dahmen Limited | Information & communication | 2025 |
| Oliver George Consulting Limited | Administrative & support | 2009 |
| Oliver Morris Ltd | Households | 2011 |
| Oliver O'Donovan Ltd | Health & social care | 2019 |
| Oliver Roth Limited | Real estate | 2022 |
| Olivers Archive Limited | Wholesale & retail | 2018 |
| Olivewood Data Technologies Ltd | Wholesale & retail | 2007 |
| Olivia & Amelia Limited | Wholesale & retail | 2023 |
| Olivia Luiza WEJ Photography Ltd | Professional & scientific | 2017 |
| Olivia Montgomerie Ltd | Extraterritorial | 2016 |
| Olivia Property 001 Limited | Construction | 2018 |
| Olko Construction Ltd | Construction | 2020 |
| Olko Properties Ltd | Real estate | 2024 |
| Ollie Bees Homes Ltd | Real estate | 2018 |
| Ollie Burditt Holdings Limited | Wholesale & retail | 2024 |
| Ollie Capital Ltd | Financial services | 2024 |
| Ollie ROY Fitness Ltd | Education | 2024 |
| Ollie's Windows Limited | Manufacturing | 2015 |
| Olliff's Architectural Antiques Limited | Construction | 2012 |
| Ollis Hair & Beauty Limited | Other services | 2023 |
| Olliv Enterprises UK Limited | Other services | 2023 |
| OLM Property Investments Ltd | Real estate | 2019 |
| Olomon Holdings Limited | Financial services | 2026 |
| Olorun Holdings Ltd | Financial services | 2022 |
| OLS Transport Ltd | Transport & logistics | 2018 |
| Olsbart Ltd | Construction | 2017 |
| Olson Limited | Administrative & support | 2018 |
| Olta Art Ltd | Information & communication | 2021 |
| Olu-Row and Sons Limited | Public administration | 2017 |
| Olulola Care Ltd | Health & social care | 2023 |
| Oluranti Adebo Ltd | Health & social care | 2015 |
| Oluwole Limited | Wholesale & retail | 2015 |
| Olveston Pre-School CIC | Health & social care | 2021 |
| Olvioh Ltd | Wholesale & retail | 2022 |