| Oakham Landscapes Limited | Other services | 2022 |
| Oakleigh 6S Limited | Manufacturing | 2024 |
| Oakleigh Engineering Limited | Professional & scientific | 2023 |
| Oakley Lane Management Limited | Professional & scientific | 2002 |
| Oakley Self Storage Ltd | Transport & logistics | 2018 |
| Oakridge (Wessex) Ltd | Arts & recreation | 2015 |
| Obaraye Consulting Limited | Information & communication | 2025 |
| OBC (Dorset) Ltd | Real estate | 2015 |
| Object Envy Ltd | Professional & scientific | 2011 |
| Obsidian Padel Ltd | Wholesale & retail | 2026 |
| Obtt Music Limited | Arts & recreation | 2025 |
| Ocat Consulting Limited | Administrative & support | 2025 |
| OCB Property Holdings Limited | Financial services | 2021 |
| Ocean Leisure Vehicles Ltd | Manufacturing | 2022 |
| Ocean Rigging Limited | Manufacturing | 2003 |
| Oceanix Limited | Information & communication | 1998 |
| Oceanside Project Management Ltd | Real estate | 2024 |
| Ockenden & Hemming Limited | Manufacturing | 2002 |
| OCM Consulting Limited | Professional & scientific | 2019 |
| Octarine Learning Consultancy Ltd | Professional & scientific | 2017 |
| Octavia Assets Ltd | Agriculture & fishing | 2013 |
| Oculus Contract Management Limited | Construction | 2022 |
| Oculus Group Ltd | Construction | 2022 |
| Oculus Roofing Ltd | Construction | 2025 |
| Odyria Limited | Information & communication | 2022 |
| OFF Court Ltd | Wholesale & retail | 2025 |
| OFF Plan Property Group Limited | Real estate | 2021 |
| Offbeat Drums Ltd | Arts & recreation | 2015 |
| OGM Construction Limited | Professional & scientific | 2023 |
| OHS Consulting Limited | Professional & scientific | 2025 |
| Oilybits Ltd | Wholesale & retail | 2011 |
| OJO Consulting Group Ltd | Professional & scientific | 2025 |
| Ojom Limited | Professional & scientific | 2022 |
| Ojsk Properties Ltd | Real estate | 2023 |
| Okari Holdings Ltd | Information & communication | 2026 |
| OKM Properties Limited | Real estate | 2017 |
| Old Green Studio Ltd | Wholesale & retail | 2022 |
| Old Salt Digital Ltd | Professional & scientific | 2019 |
| Oldbit Limited | Professional & scientific | 2022 |
| Olindo Ltd | Professional & scientific | 2022 |
| Olito Marketing Limited | Professional & scientific | 2024 |
| Olive Composites Ltd | Manufacturing | 2021 |
| Olive Grove Partners Holdings Limited | Real estate | 2026 |
| Olive Tree Homes Limited | Hospitality | 2017 |
| Oliveira Software Ltd. | Information & communication | 2022 |
| Oliver Civils Limited | Construction | 2026 |
| Olivia Close Management Company Limited | Real estate | 2013 |
| Ollieread Ltd | Information & communication | 2018 |
| Omaha Consultants Limited | Professional & scientific | 2023 |
| OMB Control Systems Solutions Ltd | Information & communication | 2014 |
| ON Surgery Ltd | Professional & scientific | 2023 |
| ON Targett Consulting Ltd | Administrative & support | 2024 |
| On-Time Plumbing Limited | Construction | 2006 |
| One Call Marine Limited | Manufacturing | 2022 |
| One Golden Nugget Limited | Other services | 2019 |
| One Leinster Square Limited | Households | 1997 |
| One Nine 58 Limited | Hospitality | 2021 |
| One Planet Development Management Limited | Financial services | 2021 |
| One Planet Developments Limited | Construction | 2019 |
| One Planet Developments Portfolio 1 Limited | Construction | 2021 |
| One-Point-One Consultants Limited | Professional & scientific | 2021 |
| Oneearth Group Limited | Construction | 2025 |
| Onesuite Consulting Ltd | Information & communication | 2024 |
| Online Tax Returns Ltd | Extraterritorial | 2024 |
| Onrepeat Ltd | Information & communication | 2026 |
| Onshop Ltd | Information & communication | 1997 |
| Onslow House (Wimborne) Management Limited | Households | 1999 |
| Onyx Lift Services Limited | Manufacturing | 2008 |
| Oops Proof Limited | Information & communication | 2025 |
| OP Contracts Ltd | Construction | 2014 |
| Opal Executive Travel Ltd | Transport & logistics | 2025 |
| Opal Financial Group (UK) Limited | Professional & scientific | 2014 |
| Opal Financial Management Limited | Professional & scientific | 2013 |
| Opal Walk Properties Limited | Real estate | 1999 |
| OPB Mortgages Limited | Financial services | 2018 |
| Open Cloud Solutions Limited | Information & communication | 2021 |
| Open Health & Safety Limited | Professional & scientific | 2022 |
| Open Sky Development Ltd | Information & communication | 2019 |
| Opti-Asset Limited | Professional & scientific | 2023 |
| Optisource Ltd | Information & communication | 2023 |
| Opus Plumbing Ltd | Construction | 2021 |
| Oracle People Solutions Ltd | Professional & scientific | 2016 |
| Orange Antlers Ltd | Information & communication | 2025 |
| Orange Plumbing Ltd | Construction | 2021 |
| Orange Synergy Limited | Real estate | 2017 |
| ORB 3D Ltd | Professional & scientific | 2019 |
| Orbis Software Limited | Information & communication | 1999 |
| Orchid Risk Management Limited | Administrative & support | 2014 |
| Orionis Limited | Wholesale & retail | 2019 |
| Orkas Consultancy Ltd | Information & communication | 2024 |
| Osborne Electrical Services Ltd | Construction | 2010 |
| OSC Plus Ltd | Other services | 2024 |
| Oscalu Ltd | Information & communication | 2016 |
| Oseyus Ltd | Information & communication | 2025 |
| OSI Technology Ltd | Information & communication | 2022 |
| OST Advisory Limited | Professional & scientific | 2026 |
| OTTA4 Limited | Households | 2003 |
| Ouch Training Team Limited | Education | 2001 |
| Outcomes Business Group Limited | Administrative & support | 2022 |
| Outer Nowhere Ltd | Information & communication | 2014 |