| Odin Consultancy Ltd | Professional & scientific | 2026 |
| ODL Drylining Ltd | Construction | 2019 |
| OEA Investments Ltd | Construction | 2024 |
| Office Re-Sale Limited | Wholesale & retail | 2009 |
| Offlead.App Ltd | Information & communication | 2025 |
| Offywoffy Ltd | Wholesale & retail | 2024 |
| Ofisdepot London Ltd | Other services | 2018 |
| Ofiy Healthcare Croydon Ltd | Health & social care | 2024 |
| Ofiy Healthcare Ltd. | Administrative & support | 2018 |
| OG Golf Apparel Ltd. | Wholesale & retail | 2019 |
| OG Juice Ltd | Other services | 2016 |
| Ogbuago Limited | Real estate | 2026 |
| Ogeps Ltd | Wholesale & retail | 2023 |
| OHA International Ltd | Wholesale & retail | 2021 |
| OHB Capital Ltd | Real estate | 2025 |
| OHB50 Ltd | Hospitality | 2025 |
| OIL - Green Life 2023 Ltd | Water & waste | 2023 |
| Okal Property Ltd | Real estate | 2018 |
| Okea Services Limited | Wholesale & retail | 2020 |
| Oksuz Ltd | Professional & scientific | 2025 |
| Okus Developments Limited | Construction | 2009 |
| Okyanus Construction Limited | Construction | 2024 |
| Olams Property Ltd Ltd | Real estate | 2025 |
| Olassco Limited | Administrative & support | 2024 |
| Old City 26 Ltd | Real estate | 2023 |
| Old London Properties Limited | Professional & scientific | 1987 |
| Old School Muscle & Fitness Ltd | Arts & recreation | 2013 |
| Oldline Education C.I.C. | Education | 2012 |
| Oldwood Limited | Professional & scientific | 2023 |
| Olflo Mechanical Ltd | Construction | 2024 |
| Olhe Properties Limited | Real estate | 2022 |
| Olilet Limited | Real estate | 2020 |
| Olive 45 Ltd | Hospitality | 2025 |
| Olive Branch VHK Limited | Wholesale & retail | 2020 |
| Olive Tree Investment London Limited | Extraterritorial | 2016 |
| Olive Tree Organic Continental Foods Ltd | Wholesale & retail | 2025 |
| Oliver & Oscar Property Management Ltd | Real estate | 2020 |
| Olivio Pizzeria Ltd | Hospitality | 2026 |
| Olmotors Ltd | Construction | 2025 |
| OLU Worrell Partnership Ltd | Real estate | 2019 |
| OLUWA-3 UK Ltd | Health & social care | 2022 |
| Olympus Qualifications Ltd | Administrative & support | 2024 |
| OM Social & Mental Health Ltd | Health & social care | 2025 |
| Omajova 1 Ltd | Professional & scientific | 2010 |
| Omar D Ltd | Health & social care | 2022 |
| Omaro Services Ltd | Information & communication | 2025 |
| Ombrica Ltd | Wholesale & retail | 2025 |
| OMEGA213 Ltd | Administrative & support | 2024 |
| Omen Barbers Ltd | Other services | 2025 |
| Omer ALI & Nilay Ltd | Real estate | 2025 |
| Omerta Construction and Services Limited | Construction | 2011 |
| Omigar Couriers Ltd | Transport & logistics | 2024 |
| OMIKO2022 Ltd | Health & social care | 2022 |
| Omnifolio C.I.C. | Information & communication | 2013 |
| Omniscient Homes Limited | Construction | 2015 |
| Omnisoft Designs Limited | Administrative & support | 2007 |
| ON Property Investment Ltd | Real estate | 2019 |
| ON the Go Fitness Ltd | Arts & recreation | 2025 |
| Onal Bookkeeping Ltd | Financial services | 2020 |
| Oncan Transport Limited | Transport & logistics | 2023 |
| Oncu Meat Limited | Wholesale & retail | 2016 |
| One Base Ltd | Information & communication | 2024 |
| One Complete Solution Ltd | Construction | 2025 |
| One CUT London Limited | Other services | 2014 |
| One Deal Ltd | Professional & scientific | 2018 |
| One Genesis Design Limited | Professional & scientific | 2024 |
| One of Nine Ltd | Information & communication | 2019 |
| One Sea Breeze Ltd | Hospitality | 2025 |
| One Space Enfield Ltd | Health & social care | 2021 |
| One Stop Structural Solutions Ltd | Professional & scientific | 2024 |
| One-Upp Consulting Limited | Information & communication | 2014 |
| ONE2ONE Safety Consultancy Ltd | Health & social care | 2015 |
| Onecashe Limited | Financial services | 1981 |
| Onemarkethub Ltd | Wholesale & retail | 2025 |
| Onesck Limited | Wholesale & retail | 2019 |
| Onestone Microcement Ltd | Construction | 2026 |
| Oneunit Entertainment Ltd | Education | 2025 |
| Oneunit Fitness Ltd | Education | 2025 |
| Onex Mania Limited | Wholesale & retail | 2025 |
| Only Sweet Delights Limited | Wholesale & retail | 2022 |
| Only the Brave Bowes Park Limited | Hospitality | 2025 |
| Only the Brave Limited | Hospitality | 2023 |
| Onur Properties LLP | | 2025 |
| Onyx Auto Limited | Wholesale & retail | 2022 |
| Open Arms Lodge Limited | Hospitality | 2025 |
| Open House Estates Ltd | Real estate | 2024 |
| Open Woods CIC | Information & communication | 2026 |
| Openmar Investments LLP | | 2013 |
| Openmar LLP | | 2013 |
| Ophthalmological Products Trade and Industry Conference(The) | Other services | 1983 |
| Optamate AI Ltd | Information & communication | 2026 |
| Optigrowth Pro Limited | Professional & scientific | 2025 |
| Optimal Scaffolding Solutions Ltd | Construction | 2025 |
| Optimam Complex Care Services Limited | Administrative & support | 2021 |
| Optimam Limited | Health & social care | 2015 |
| Optimam Property Services Ltd | Real estate | 2025 |
| Optimum Drinks Ltd | Wholesale & retail | 2025 |
| Optimum Pharm Ltd | Health & social care | 2019 |
| Optimum Support Services Limited | Administrative & support | 2026 |
| Optiwatch Ltd | Administrative & support | 2015 |