| Oak Pharma Ltd | Public administration | 2022 |
| Oak Roofing Services Limited | Construction | 2021 |
| Oak Tree Upholstery Limited | Manufacturing | 2008 |
| Oakbridge Limited | Real estate | 2024 |
| Oaken Contractors Ltd | Construction | 2019 |
| Oakhinge Limited | Manufacturing | 1979 |
| Oaklands Trading Company Limited | Wholesale & retail | 2003 |
| Oaklea House Ltd | Wholesale & retail | 2024 |
| Oakmount Close Pelsall Limited | Real estate | 1982 |
| Oakstell Ltd | Health & social care | 2025 |
| Oakswood Construction & Groundworks Ltd | Construction | 2008 |
| Oaktan Belmont Ltd | Wholesale & retail | 2023 |
| Oakvalley Limited | Real estate | 2003 |
| Oakwood Community Care Ltd | Health & social care | 2026 |
| Oakwood Digital Holdings Limited | Financial services | 2025 |
| Oakwood Solutions Limited | Information & communication | 2003 |
| OAL Consultancies Limited | Information & communication | 2018 |
| OAL Invest Limited | Real estate | 2024 |
| OAO Group Ltd | Other services | 2020 |
| Oasis Professionals Ltd | Professional & scientific | 2024 |
| Oberai Investments Ltd | Financial services | 2022 |
| Oberai Ltd | Health & social care | 2020 |
| Oberai Properties Ltd | Real estate | 2022 |
| Oberon Artist Management Ltd | Information & communication | 2023 |
| Obreka Ltd | Professional & scientific | 2010 |
| Obssesive RBL Collection Ltd | Wholesale & retail | 2021 |
| OC Racing Limited | Administrative & support | 2016 |
| Occasional Event Support Limited | Professional & scientific | 2024 |
| Occuvita Limited | Education | 2017 |
| Ocean Blue Spa Centre Limited | Other services | 2024 |
| Ocean Express Travel & Tours Ltd | Administrative & support | 2026 |
| Ocean Green Property Ltd | Real estate | 2020 |
| Ocean Interiors (Midlands) Ltd | Construction | 2011 |
| Ocean Medica Limited | Health & social care | 2018 |
| Ocean Twenty-Five Limited | Professional & scientific | 2009 |
| Oceanbeck Ltd | Wholesale & retail | 2012 |
| Oceanic Real Estate Limited | Real estate | 2022 |
| Oceanic Windows Limited | Construction | 2018 |
| Octabuild Construction Ltd | Construction | 2025 |
| Octavian Healthcare Limited | Wholesale & retail | 2017 |
| Octex Limited | Wholesale & retail | 2021 |
| Octopus INK Publishing Ltd | Information & communication | 2026 |
| Oculy Limited | Manufacturing | 2019 |
| OD Trans Limited | Transport & logistics | 2018 |
| Odis Labs Ltd | Information & communication | 2020 |
| ODL Group Ltd | Administrative & support | 2024 |
| Odyssey Real Estate Limited | Construction | 2015 |
| OE Systems Limited | Manufacturing | 2010 |
| Ofdas Limited | Wholesale & retail | 2018 |
| OFF Grid Gourmet Limited | Hospitality | 2016 |
| Offgrid Games Ltd | Information & communication | 2020 |
| Office Watercoolers Limited | Administrative & support | 2001 |
| Officialdealsuk Ltd | Wholesale & retail | 2022 |
| Officialkickkzuk Ltd | Wholesale & retail | 2024 |
| Offinba Ltd | Health & social care | 2022 |
| Offtherail Ltd | Wholesale & retail | 2026 |
| Ofita Ltd | Information & communication | 2015 |
| OFW Online Ltd | Wholesale & retail | 2023 |
| OH My Dog Shop Ltd | Wholesale & retail | 2022 |
| OH SO Sweet Limited | Wholesale & retail | 2016 |
| Ohemaa Unique Hair & Salon Ltd | Other services | 2024 |
| Oionssaw Ltd | Manufacturing | 2025 |
| OJN Building & Roofing Ltd | Construction | 2026 |
| Okbu Store Limited | Wholesale & retail | 2025 |
| Okey Services Limited | Health & social care | 2014 |
| Olak Care Solutions Limited | Health & social care | 2024 |
| Olat Healthcare Ltd | Health & social care | 2022 |
| Old Hall Court (West Midlands) Management Limited | Households | 1998 |
| Old Hall Peoples Partnership | Financial services | 2000 |
| Old Park Road Limited | Real estate | 2004 |
| Old Park Station Limited | Professional & scientific | 2024 |
| Old Skool Mini's Ltd | Wholesale & retail | 2021 |
| Oldara Limited | Wholesale & retail | 2002 |
| Olive & OUD Limited | Wholesale & retail | 2025 |
| Olive Healthcare Ltd | Administrative & support | 2025 |
| Olive Support Solutions Ltd | Real estate | 2024 |
| Oliver U.Tallett Limited | Wholesale & retail | 1939 |
| Oliver U.Tallett(Holdings)Limited | Wholesale & retail | 1960 |
| Oliver Wellings Designs Ltd | Professional & scientific | 2023 |
| Olives Healthcare Ltd | Health & social care | 2018 |
| Olli Construction Ltd | Construction | 2018 |
| OM SOM Ltd | Professional & scientific | 2024 |
| Omah Film Services Ltd | Professional & scientific | 2025 |
| Omar Afridi Consultancy Ltd | Administrative & support | 2025 |
| Omarakhtar Tech Ltd | Manufacturing | 2026 |
| Omega Bathrooms & DIY Ltd | Wholesale & retail | 2024 |
| Omega Building Supplies Ltd | Wholesale & retail | 2023 |
| Omega Commodities Ltd | Financial services | 2018 |
| Omega Creative Builders Limited | Construction | 2024 |
| Omega Property Solutions Ltd | Hospitality | 2016 |
| Omega Renewables Limited | Professional & scientific | 2019 |
| OMI Traders Ltd | Wholesale & retail | 2025 |
| Omni Education Consultancy Ltd | Education | 2022 |
| Omni Workforce Solutions Ltd | Administrative & support | 2026 |
| Omnibio Ltd | Professional & scientific | 2023 |
| Omnium Electrical Limited | Construction | 2023 |
| ON ALL Floors Bespoke Flooring Ltd | Construction | 2011 |
| ON Demand Deals Ltd | Wholesale & retail | 2025 |
| ON Gaurd Security Services Ltd | Administrative & support | 2025 |
| ON Road Assist Ltd | Wholesale & retail | 2024 |