| Oaktree Storage & Logistics Ltd | Transport & logistics | 2026 |
| Oakview Cars Limited | Transport & logistics | 2013 |
| Observer Media Group Ltd | Information & communication | 2018 |
| Obsessive Hair Salon Ltd | Other services | 2024 |
| Occupy World Outreach Ministries | Other services | 2008 |
| Ocean Air Conditioning Limited | Administrative & support | 2004 |
| Ocean Interiors (UK) Limited | Construction | 2001 |
| Ocean Procurement Services Limited | Wholesale & retail | 2009 |
| Ocean Spices Limited | Hospitality | 2002 |
| Ocean Turtle Diving Limited | Wholesale & retail | 2004 |
| Ocean Valet Co Ltd | Administrative & support | 2023 |
| Octa Partners Ltd | Professional & scientific | 2025 |
| Octobridge Limited | Professional & scientific | 2014 |
| Ocular Investments Limited | Extraterritorial | 2012 |
| Odin Careers Ltd | Administrative & support | 2024 |
| Odyssey Consult Limited | Professional & scientific | 2016 |
| Odyssey Lets Ltd | Real estate | 2014 |
| OFF Trail Ltd | Wholesale & retail | 2024 |
| OG3 Management Limited | Health & social care | 2022 |
| OJ Consultancy Ltd | Information & communication | 2017 |
| OJ Motors Services Ltd | Wholesale & retail | 2020 |
| Ojak Consultancy Limited | Professional & scientific | 2012 |
| Okatnhyie Prime Properties Limited | Real estate | 2026 |
| Olasport Ltd | Manufacturing | 2025 |
| Olay Services Limited | Health & social care | 2021 |
| Old Basing Archers Limited | Arts & recreation | 2023 |
| Old Basing Barn Limited | Real estate | 2017 |
| Old London Bricks Properties Ltd | Real estate | 2024 |
| Olek Transport Ltd | Transport & logistics | 2016 |
| Olimedic Limited | Health & social care | 2016 |
| Olio International Limited | Information & communication | 2009 |
| Olive Blossom Bridal Ltd | Wholesale & retail | 2020 |
| Olive Tree Renovations Limited | Construction | 2013 |
| Oliver Featherman Ltd | Professional & scientific | 2008 |
| Oliver Newin Events Limited | Professional & scientific | 2009 |
| Oliver Wheels Ltd | Wholesale & retail | 2025 |
| Oliver Wood Architects Ltd | Professional & scientific | 2023 |
| Olivia & Serah IT Solutions Ltd | Information & communication | 2018 |
| Olma Solutions Limited | Information & communication | 2017 |
| OLS & OLS Ltd | Wholesale & retail | 2020 |
| Omakase Japanese Restaurant Ltd | Hospitality | 2024 |
| Omegahp Ltd | Wholesale & retail | 2023 |
| Omni Cleaning Services Limited | Administrative & support | 2017 |
| Omni Interim Management and Consultancy Ltd | Professional & scientific | 2024 |
| Omniana Limited | Information & communication | 2019 |
| Omsai Laxmi Ltd | Hospitality | 2018 |
| Omsai Limited | Information & communication | 2008 |
| ON Feet UK Limited | Wholesale & retail | 2025 |
| ON Point C Ltd | Professional & scientific | 2025 |
| ON Target Media Ltd | Information & communication | 2007 |
| ON Time Care Ltd | Health & social care | 2016 |
| ON Your Bike Cycle Training Ltd | Education | 2020 |
| Once for ALL Limited | Administrative & support | 2018 |
| One Bentley Square Ltd | Real estate | 2024 |
| One Call Systems Limited | Manufacturing | 2021 |
| One Fifty Five Ltd | Professional & scientific | 2017 |
| One Saint Cross Limited | Households | 2016 |
| One to Five Winchester Road Residents Company Limited | Households | 2014 |
| Onediem Ltd | Hospitality | 2020 |
| Oner Scaffolding Limited | Construction | 2025 |
| Onkaar Pandora Box Ltd | Wholesale & retail | 2019 |
| Online Goods Ltd | Wholesale & retail | 2023 |
| Online Insight Ltd | Information & communication | 2017 |
| Onlyscience Ltd | Information & communication | 2024 |
| ONS Investments Ltd | Financial services | 2019 |
| Ontime.Oncost Ltd | Information & communication | 2019 |
| Ontrack Courier Services Ltd | Transport & logistics | 2018 |
| Opal Grove Property Ltd | Real estate | 2024 |
| Opal Reality Limited | Real estate | 2023 |
| Opencraft Consulting Ltd | Information & communication | 2025 |
| Opendoor Print Limited | Manufacturing | 2007 |
| Operational & Leadership Solutions Ltd | Professional & scientific | 2017 |
| Oppersdorff Colour Ltd | Information & communication | 2026 |
| Opratives Ltd | Administrative & support | 2026 |
| Oprocon Limited | Professional & scientific | 2007 |
| Opscomms Consulting Ltd | Professional & scientific | 2013 |
| Optima Property Ltd | Real estate | 2019 |
| Optimal Sports Therapy Centre Limited | Arts & recreation | 2022 |
| Optimized Consultancy Ltd | Professional & scientific | 2013 |
| Optimized Property Services Ltd | Real estate | 2019 |
| Optimus Patents Limited | Professional & scientific | 2006 |
| Options Plumbing and Heating Ltd | Construction | 2021 |
| ORA Music Limited | Information & communication | 2026 |
| Oracle Special Metals Ltd | Wholesale & retail | 2020 |
| Orange Motorsport Limited | Arts & recreation | 2018 |
| Orbel Services Ltd | Construction | 2003 |
| Orchard Consulting (GB) Limited | Professional & scientific | 2002 |
| Ordell Arms Freehold Limited | Households | 2012 |
| Ordinary Media Ltd | Information & communication | 2020 |
| Orestone Controls Limited | Construction | 2015 |
| Organic Botanics Ltd | Wholesale & retail | 2026 |
| Oriental Basingstoke Ltd | Hospitality | 2018 |
| Oriental Medic & Herbs Ltd | Health & social care | 2015 |
| Oriental Pearl Food Ltd | Hospitality | 2024 |
| Origin Storage Holdings Group Ltd | Extraterritorial | 2022 |
| Origin Storage Holdings Ltd | Manufacturing | 2019 |
| Origin Storage Limited | Manufacturing | 2000 |
| Origin Thread Ltd | Wholesale & retail | 2021 |
| Original Software Limited | Extraterritorial | 1997 |
| Origsoft Limited | Extraterritorial | 1999 |