| ON Site Plus Limited | Health & social care | 2005 |
| ON TAP Comms Limited | Professional & scientific | 2012 |
| ON Trend Fabrics Ltd | Wholesale & retail | 2018 |
| Onboard Recruitment Ltd | Administrative & support | 2013 |
| Once Upon a Tea Room Limited | Hospitality | 2018 |
| One Banana Solutions Limited | Information & communication | 2024 |
| One for One's Beauty Limited | Health & social care | 2002 |
| One Forty One Limited | Households | 1996 |
| One Horse Property Limited | Real estate | 2022 |
| One Jersey Place Limited | Construction | 2025 |
| One Loft Race Ltd | Agriculture & fishing | 2025 |
| One Quarter Ltd | Professional & scientific | 2019 |
| One Stop Shop Business Ltd | Wholesale & retail | 2024 |
| One to One Recruitment Ltd | Administrative & support | 2005 |
| One With Nutrition Ltd | Other services | 2019 |
| One World Healthcare Communications Ltd | Professional & scientific | 2025 |
| ONE7 Projects Limited | Construction | 2024 |
| Oneills Decorating Limited | Construction | 2019 |
| Onelite Clothing Ltd | Wholesale & retail | 2018 |
| Onewell Solutions Limited | Professional & scientific | 2026 |
| Onfocus Tech Ltd | Information & communication | 2018 |
| Only Vans Entertainment Ltd | Wholesale & retail | 2025 |
| Onlycoaching Limited | Education | 2023 |
| Ontic Engineering & Manufacturing UK Limited | Manufacturing | 2008 |
| Onword PT Ltd | Wholesale & retail | 2025 |
| Onyss Ltd | Information & communication | 2025 |
| Ooak Consultancy Limited | Professional & scientific | 2022 |
| Oodles Precious Metals Ltd | Wholesale & retail | 2020 |
| Ootadootas Limited | Wholesale & retail | 2025 |
| Opal Production Limited | Administrative & support | 2009 |
| Opal Wild Limited | Wholesale & retail | 2023 |
| Opemed 2020 Ltd. | Professional & scientific | 2020 |
| Open Door Logistics Ltd | Transport & logistics | 2014 |
| Open Season Limited | Professional & scientific | 2026 |
| Open Water Swimming Limited | Arts & recreation | 2014 |
| Open Wide Medical Ltd. | Health & social care | 2014 |
| Opengov Limited | Information & communication | 2017 |
| Opex Fitness Manchester Ltd | Arts & recreation | 2018 |
| Opiyo Limited | Administrative & support | 2010 |
| Oppidanmusic Ltd | Arts & recreation | 2024 |
| Opportuni AI Ltd | Information & communication | 2020 |
| Optic House Limited | Financial services | 2018 |
| Optimal Living (Kent) Limited | Health & social care | 2017 |
| Optimal Living (Luton) Limited | Health & social care | 2010 |
| Optimal Living Ltd | Health & social care | 1980 |
| Optimal Management Development Limited | Professional & scientific | 2003 |
| Optimise Strategic Land Limited | Professional & scientific | 2023 |
| Optimism Health Group II Limited | Financial services | 2022 |
| Optimum Elearning Ltd | Education | 2025 |
| Optimum Orthodontics Ltd | Health & social care | 2017 |
| Optimum Property Ltd | Real estate | 2018 |
| Optimum Student Support Ltd | Education | 2019 |
| Opus 55 Limited | Professional & scientific | 2012 |
| OR Leasing Co Ltd | Real estate | 2025 |
| OR Lettings Ltd | Real estate | 2023 |
| OR Operations Ltd | Hospitality | 2025 |
| OR Power Ltd | Administrative & support | 2025 |
| Orabella Ltd | Other services | 2016 |
| Orange Appointments Ltd | Administrative & support | 2015 |
| Orange House Construction Limited | Construction | 2010 |
| Orange Tree Toys International Limited | Financial services | 2018 |
| Orange Tree Toys Ltd | Administrative & support | 2000 |
| Orangeyoga Education Limited | Education | 2024 |
| Orangeyoga Events Limited | Administrative & support | 2024 |
| Orangeyoga Studio CIC | Education | 2020 |
| Orangeyoga Therapy Limited | Education | 2024 |
| ORB BIA Ltd | Administrative & support | 2024 |
| Orbital Limited | Professional & scientific | 1974 |
| Orchard & Banks Ltd | Water & waste | 2015 |
| Orchard Dog Grooming Ltd | Other services | 2019 |
| Orchard Drive Management Ltd | Households | 2013 |
| Orchard Projects Limited | Households | 1994 |
| Orchard Villas Danford Ltd | Real estate | 2025 |
| Ordis Management Ltd | Information & communication | 2025 |
| Ordonomic Limited | Information & communication | 2026 |
| Oremont Partners Ltd | Administrative & support | 2026 |
| Organara Ltd | Manufacturing | 2021 |
| Organic Family Chiropractic Ltd | Health & social care | 2021 |
| Organik Ltd | Extraterritorial | 2001 |
| Organise for YOU Ltd | Administrative & support | 2011 |
| Oriel Collections Limited | Administrative & support | 2003 |
| Oriel House Freehold Management Limited | Real estate | 2018 |
| Oriel Villas (Cheltenham) Limited | Households | 2012 |
| Orient-Asia Foods Ltd | Wholesale & retail | 2019 |
| Oriental Food Stores 1 Limited | Wholesale & retail | 2023 |
| Origin (Bristol) Ltd | Wholesale & retail | 1992 |
| Origin 2 Limited | Administrative & support | 2021 |
| Origin Investments Ltd | Financial services | 2005 |
| Origin Property Management Ltd | Real estate | 2004 |
| Original Consultants Ltd | Information & communication | 2020 |
| Orman Legal Limited | Professional & scientific | 2010 |
| ORO London Ltd | Health & social care | 2013 |
| Orpheus Books Limited | Information & communication | 1993 |
| Orsip Limited | Information & communication | 2022 |
| Orso Publishing Ltd | Information & communication | 2022 |
| Ortho House Limited | Health & social care | 2016 |
| Orthod Midco Limited | Manufacturing | 2016 |
| Orthostyle Limited | Health & social care | 2006 |
| Orvantis Jewellery Ltd | Wholesale & retail | 2026 |
| OS (Realisations) Limited | Professional & scientific | 2007 |