| O:Ad Studio Limited | Professional & scientific | 2018 |
| OA Engineering & Innovations Ltd | Professional & scientific | 2024 |
| OAC Commercial Ltd | Construction | 2023 |
| OAC Supplies Ltd | Administrative & support | 2023 |
| Oadby TV Repair Specialists Ltd | Other services | 2026 |
| OAH Consultancy Ltd | Education | 2025 |
| Oahrobert Limited | Manufacturing | 2023 |
| Oak & Acorn Ltd | Manufacturing | 2021 |
| Oak & Hammer Limited | Construction | 2025 |
| Oak and Stone Landscaping Ltd | Professional & scientific | 2025 |
| Oak Archaeology Wales CIC | Education | 2025 |
| Oak Electrical Installations Ltd | Construction | 2022 |
| Oak FS Ltd | Financial services | 2024 |
| Oak House Care Limited | Health & social care | 2009 |
| Oak Room Associates Ltd | Households | 2024 |
| Oak Strength Limited | Wholesale & retail | 2023 |
| Oakapple Flat Management Limited | Households | 1992 |
| Oakes Interiors Ltd | Manufacturing | 2024 |
| Oakes Shaw Creatives Ltd | Professional & scientific | 2024 |
| Oakfield Barbers Limited | Other services | 2025 |
| Oakfield House 2017 Limited | Households | 2017 |
| Oakfield Mews Management Company Limited | Households | 2003 |
| Oakfield Property Management Ltd | Real estate | 2021 |
| Oakfields Group Limited | Health & social care | 2026 |
| Oakhill Associates Limited | Professional & scientific | 2009 |
| Oakhill Court (Freehold) Limited | Households | 2010 |
| Oakhurst Property Limited | Real estate | 2009 |
| Oaking Developments (College Avenue) Limited | Construction | 2017 |
| Oakland College Ltd | Education | 2024 |
| Oaklands Developments Ltd | Construction | 2015 |
| Oakleigh House Developments Limited | Construction | 2015 |
| Oakleigh House Farm Ltd | Agriculture & fishing | 2020 |
| Oakley Brothers Legal Services Limited | Professional & scientific | 2019 |
| Oakley Family Holdings Limited | Professional & scientific | 2024 |
| Oakma Investments UK Limited | Real estate | 2023 |
| Oakmare Ltd | Real estate | 2026 |
| Oakmont Venture Group Limited | Financial services | 2025 |
| Oakmore Services LLP | | 2023 |
| Oakread Limited | Construction | 2017 |
| Oakridge Cloud Limited | Information & communication | 2025 |
| Oakridge Extensions Ltd | Construction | 2024 |
| Oakridge Retail Ltd | Wholesale & retail | 2025 |
| Oakross Limited | Health & social care | 2020 |
| Oakspring Property Investments Ltd | Real estate | 2025 |
| Oaktree Court RTM Company Limited | Households | 2017 |
| Oakvest Ventures Limited | Construction | 2026 |
| Oakville (South Wales) Limited | Financial services | 2010 |
| Oakville Care Limited | Health & social care | 2003 |
| Oakville Resorts Ltd | Hospitality | 2025 |
| Oakwest Ltd | Financial services | 2018 |
| Oakwey Limited | Administrative & support | 2018 |
| Oakwood Healthcare Limited | Administrative & support | 2015 |
| Oakwood Road SPV Ltd | Real estate | 2021 |
| OAM 2020 Limited | Health & social care | 2008 |
| Oana Victoria Services Ltd | Health & social care | 2018 |
| Oarcr Services Ltd | Transport & logistics | 2026 |
| Oasis Brickwork Limited | Construction | 2018 |
| Oasis Capital Finance Limited | Financial services | 2019 |
| Oasis Capital Group Limited | Professional & scientific | 2016 |
| Oasis Cleaning Services Limited | Administrative & support | 2015 |
| Oasis Crew Limited | Administrative & support | 2024 |
| Oasis Flex Ltd | Wholesale & retail | 2025 |
| Oasis NO. 1 LLP | | 2006 |
| Oasis NO. 2 LLP | | 2006 |
| Oasis NO. 3 LLP | | 2006 |
| Oasis Play Tech Limited | Professional & scientific | 2026 |
| Oasis Sunbed Hire Ltd | Administrative & support | 2017 |
| Oasis Vigour Food Ltd | Manufacturing | 2024 |
| OAT & Bean Ltd | Hospitality | 2023 |
| Oath Properties Ltd | Hospitality | 2025 |
| Oatt Ltd | Wholesale & retail | 2025 |
| OB Investments Management Limited | Professional & scientific | 2012 |
| Obaidtech Limited | Wholesale & retail | 2025 |
| Obair Relocations Ltd | Hospitality | 2022 |
| Obals Ltd | Health & social care | 2019 |
| OBC Scaffolding Ltd | Construction | 2024 |
| OBD MOT Centre Ltd | Wholesale & retail | 2025 |
| Obenyx Ltd | Information & communication | 2025 |
| Obesl Mart Ltd | Wholesale & retail | 2025 |
| Obhealt Ltd | Public administration | 2022 |
| Obhi Trading Ltd | Wholesale & retail | 2025 |
| OBI Store Ltd | Wholesale & retail | 2024 |
| Obichi Ltd | Information & communication | 2021 |
| Obireen UK Limited | Manufacturing | 2025 |
| Object 1 Solutions Limited | Information & communication | 2020 |
| Object Matrix Limited | Wholesale & retail | 2003 |
| Objective Technologies Ltd | Information & communication | 2008 |
| Objekt Events Ltd | Information & communication | 2023 |
| Oblisystech Ltd | Information & communication | 2025 |
| Oblivion Displays Limited | Professional & scientific | 2025 |
| Oblivion Properties Limited | Real estate | 2026 |
| Obrenza Ltd | Information & communication | 2019 |
| OBS Lofts and Extensions Ltd | Construction | 2019 |
| OBS Style Ltd | Wholesale & retail | 2019 |
| Obsessed Acquisition 1 Ltd | Professional & scientific | 2025 |
| Obsessivecompulsivegymwear Ltd | Wholesale & retail | 2017 |
| Obsidian Protection Services Ltd | Administrative & support | 2025 |
| Obsidian Threads Ltd | Wholesale & retail | 2025 |
| Obzidian Holdings Ltd | Construction | 2024 |
| Obzor Group PC | | 2018 |