| Oakview Tree Services Ltd | Administrative & support | 2025 |
| Oakwood Education | Education | 2007 |
| Oakwood Estates Management Ltd | Real estate | 2025 |
| Oakwood Investment Group Ltd | Real estate | 2024 |
| Oakwood Letting & Estates Limited | Real estate | 2016 |
| Oakwood Repair & Maintenance Limited | Other services | 2018 |
| Oandlm Limited | Real estate | 2012 |
| OAR Property Ltd | Real estate | 2024 |
| Oarco Limited | Administrative & support | 2021 |
| Oasis & Olive Limited | Information & communication | 2026 |
| Oasis Inc Limited | Real estate | 2016 |
| Oasis Meridian Limited | Information & communication | 2026 |
| Oasis Primary Care Network Ltd | Health & social care | 2025 |
| Oasis Resourcing Limited | Administrative & support | 2022 |
| Ob-Vious-Lyyy Limited | Arts & recreation | 2019 |
| OBA45 Limited | Real estate | 2020 |
| OBC Wholesale Ltd | Wholesale & retail | 2019 |
| Obdatasolutions Ltd | Information & communication | 2025 |
| OBL Civils Ltd | Construction | 2020 |
| Obra ZA Ltd | Wholesale & retail | 2025 |
| OCD Clinic Ltd | Administrative & support | 2026 |
| Ocean and Air Logistics Ltd | Transport & logistics | 2018 |
| Ocean Blow Limited | Wholesale & retail | 2019 |
| Ocean Contract Cleaning Limited | Other services | 2012 |
| Ocean Group Holdings Limited | Other services | 2009 |
| Ocean Online Limited | Wholesale & retail | 2023 |
| Ocean Security Solutions Limited | Other services | 1999 |
| Ocean Select Ltd | Wholesale & retail | 2023 |
| Ocean Wild International Limited | Wholesale & retail | 2017 |
| Oceanic Glow Limited | Wholesale & retail | 2025 |
| Ochee Limited | Manufacturing | 2008 |
| Ocimum Healthcare Ltd | Health & social care | 2023 |
| Octagon Construction Solutions Limited | Construction | 2019 |
| Octane Performance Ltd | Wholesale & retail | 2020 |
| Octive Limited | Professional & scientific | 2022 |
| Ocustay Ltd | Real estate | 2025 |
| Odedra & C M Charles Ltd | Financial services | 2014 |
| Odpk Limited | Health & social care | 2016 |
| ODR Holdings Ltd | Financial services | 2025 |
| ODT Solutions Ltd | Transport & logistics | 2025 |
| Odyssey Education Ltd | Information & communication | 2015 |
| OEM Technologies Limited | Professional & scientific | 2011 |
| OFF Sight Entertainment Ltd | Professional & scientific | 2023 |
| Official GVB Limited | Wholesale & retail | 2021 |
| Offley Autocare Ltd | Wholesale & retail | 2025 |
| Offley Motors Ltd | Wholesale & retail | 2024 |
| Offpitt Ltd | Wholesale & retail | 2025 |
| OG Property Solutions Ltd | Real estate | 2020 |
| Oggasoga Ltd | Real estate | 2022 |
| Ogunleye Enterprises Ltd. | Information & communication | 2015 |
| OGY Transport Ltd | Transport & logistics | 2026 |
| OH SO Easy Homes Limited | Real estate | 2022 |
| Ohana DRY Cleaning & Tailoring Limited | Other services | 2021 |
| Ohms & Bytes Installations Ltd | Construction | 2014 |
| Ohogasengineering. Ltd | Professional & scientific | 2024 |
| Ohsho Ltd | Wholesale & retail | 2023 |
| Ohyh LLP | | 2006 |
| OIC Estates Ltd | Real estate | 2026 |
| Oina Ltd | Information & communication | 2025 |
| Oishin Ltd | Hospitality | 2015 |
| OJ Transports UK Ltd | Transport & logistics | 2025 |
| Ojey Care Solutions Limited | Health & social care | 2015 |
| Okafor and Dawkins Legal Consulting Services Ltd | Education | 2023 |
| Okaysi Limited | Wholesale & retail | 2021 |
| Oketonna Ltd | Health & social care | 2019 |
| Okholdings Limited | Real estate | 2018 |
| Okka (ST Albans) Limited | Hospitality | 2017 |
| Okobi Limited | Wholesale & retail | 2020 |
| Okolie Associates Limited | Health & social care | 2018 |
| Oktavated Ltd | Information & communication | 2019 |
| OLA Accountancy Holding Ltd | Financial services | 2026 |
| OLA Accountancy Ltd | Professional & scientific | 2023 |
| OLA Estate Ltd | Administrative & support | 2025 |
| OLA Optimal Advice Ltd | Administrative & support | 2021 |
| Olaru Construction Ltd | Construction | 2009 |
| Old Hall Court (Whitwell) Limited | Households | 2000 |
| Old Hunters Lodge (Whipsnade) Ltd | Hospitality | 2020 |
| Old Mill (UK) Ltd | Real estate | 2013 |
| Old Street Flats Limited | Households | 1997 |
| Old Town Coffee Limited | Hospitality | 2020 |
| Oldham Services Ltd | Real estate | 2019 |
| Olee Trans Ltd | Transport & logistics | 2018 |
| Olgert Management Ltd | Households | 2025 |
| Olison Group Limited | Wholesale & retail | 2025 |
| Olive Developments Ltd | Real estate | 2022 |
| Olive Heritage Consultancy Limited | Administrative & support | 2012 |
| Oliver AG 2010 Limited | Professional & scientific | 2010 |
| Oliver Agriculture Limited | Wholesale & retail | 2008 |
| Oliver Landpower Limited | Wholesale & retail | 2008 |
| Oliver LP 2010 Limited | Professional & scientific | 2010 |
| Oliver Twist Consultancy Limited | Professional & scientific | 2024 |
| Olivers Catering Services Ltd | Hospitality | 2025 |
| Olivers Way (Hertford Heath) Management Company Limited | Households | 2007 |
| Olives Cleaning Services Limited | Administrative & support | 2023 |
| Ollie Marchon Ltd | Arts & recreation | 2015 |
| Olney Care Homes Limited | Health & social care | 2004 |
| Oloriade Beauty Salon Limited | Other services | 2026 |
| Oludotun Construction Ltd | Construction | 2023 |
| Oluu Ltd | Real estate | 2019 |
| OM Beauty Limited | Other services | 2013 |