| OKI Inspection Services Ltd | Professional & scientific | 2025 |
| OKM Communications Ltd | Professional & scientific | 2021 |
| Okomi Logistics Ltd | Transport & logistics | 2024 |
| OKTOBER6 Ltd | Information & communication | 2015 |
| Olaiya African Food Store Ltd. | Households | 2009 |
| Olaru Drive Limited | Transport & logistics | 2021 |
| Olasumbo Odukoya Ltd | Information & communication | 2018 |
| Olatej & Paul Ltd | Construction | 2023 |
| Old Baby Ltd | Wholesale & retail | 2024 |
| Old Basing Pharma Ltd | Wholesale & retail | 2026 |
| Old Bedford Properties Limited | Real estate | 2021 |
| Old Brewery Salisbury Management Company Limited | Households | 2022 |
| Old Chesterton Building Freehold Limited | Real estate | 2014 |
| Old Courthouse Management Company Limited | Households | 1980 |
| Old Dairy Mews Management Company Limited | Households | 2000 |
| Old Laundry Mews Management Company Limited | Households | 2011 |
| Old Quay Meadow Management Limited | Households | 2017 |
| Old School Rise (Monmouth) Management Company Limited | Households | 2012 |
| Old Smiths Yard Management Company Limited | Households | 2005 |
| Old Swan Wharf (Management) Company Limited | Households | 1995 |
| Oldgate Investments Limited | | 2005 |
| Oled and Co Ltd | Administrative & support | 2025 |
| Olera Ltd | Other services | 2018 |
| OLH Beauty Limited | Other services | 2020 |
| OLIV-3 Solutions Limited | Extraterritorial | 2012 |
| Olive Cars Ltd | Wholesale & retail | 2019 |
| Olive Tree Day Nursery and School Ltd | Education | 2021 |
| Oliveheal Healthcare Ltd | Health & social care | 2021 |
| Oliveira Care Ltd | Wholesale & retail | 2020 |
| Oliveiras Construction Limited | Construction | 2023 |
| Oliver House and Barclay Grange Limited | Households | 2001 |
| Oliver Plumbing and Heating Limited | Construction | 2019 |
| Olivewood Estates Limited | Real estate | 2021 |
| Olivia Apparel Limited | Wholesale & retail | 2026 |
| Olivia House Management Limited | Households | 2018 |
| Olivier Court (Bedford) Management Company Limited | Households | 1988 |
| Olivier Hardware Ltd | Wholesale & retail | 2026 |
| Ollier Associates Limited | Professional & scientific | 2002 |
| Olly Pro Build Ltd Limited | Construction | 2020 |
| Olly-Links Ltd | Administrative & support | 2025 |
| Ollyniy Limited | Public administration | 2021 |
| Olman Ltd | Information & communication | 2009 |
| OLR (UK) Limited | Professional & scientific | 2008 |
| OLR Group Limited | Information & communication | 2016 |
| OLR Integrated Limited | Information & communication | 2023 |
| Olympia Courier Ltd | Transport & logistics | 2021 |
| Olympia Partners Employee Nominee Company Limited | Financial services | 2025 |
| Olympia Partners Limited | Financial services | 2019 |
| Olympus LV Ltd | Real estate | 2014 |
| OM Durham Ltd | Wholesale & retail | 2023 |
| OM Krupa Limited | Real estate | 2024 |
| Omac Building Services Ltd | Public administration | 2022 |
| Oman Computers Limited | Information & communication | 1997 |
| Omatics Limited | Manufacturing | 1991 |
| Omdd Property Limited | Real estate | 2022 |
| Omega AP Ltd | Professional & scientific | 2025 |
| Omega 10 Limited | Construction | 2018 |
| Omega 16 Limited | Construction | 2018 |
| Omega 7 Limited | Construction | 2017 |
| Omega ACC Properties Ltd | Real estate | 2022 |
| Omega Accountant and Taxation Ltd | Professional & scientific | 2024 |
| Omega Accountants Limited | Professional & scientific | 2001 |
| Omega Force Ltd | Administrative & support | 2012 |
| Omega Land Holdings Limited | Real estate | 2016 |
| Omega Racing Limited | Administrative & support | 2011 |
| Omega Site Personnel (London) Limited | Administrative & support | 2013 |
| Omfys Technologies UK Ltd | Information & communication | 2025 |
| Omguruom Investments Limited | Real estate | 2021 |
| OMI Security Ltd | Administrative & support | 2023 |
| Omiiko Health Care Services Ltd | Health & social care | 2023 |
| Omkara Ventures Ltd | Information & communication | 2025 |
| Omnedoc-Health Ltd | Health & social care | 2020 |
| Omniscient Limited | Hospitality | 2000 |
| Omolade Ltd | Wholesale & retail | 2023 |
| ON Cloud Pine Ltd | Wholesale & retail | 2020 |
| ON Site Projects Ltd | Real estate | 2012 |
| ON the 8TH Day Management Company Limited | Households | 2003 |
| ON the Move Lifestyle Ltd | Hospitality | 2022 |
| ON the Road Tours Ltd | Administrative & support | 2025 |
| ON the Scene LDN Limited | Hospitality | 2024 |
| ON Time Sped Trans Ltd | Transport & logistics | 2021 |
| ON Your Game Laser Treatment Clinic Ltd | Health & social care | 2022 |
| ON Your Game Physiotherapy Ltd | Health & social care | 2022 |
| Once ITS Organised Ltd | Transport & logistics | 2024 |
| Once Upon a Time (Croydon) Ltd | Education | 2016 |
| Oncodiagnostics Ltd | Health & social care | 2024 |
| One Academy Ltd | Other services | 2021 |
| One and Only Limited | Wholesale & retail | 2026 |
| One Bilbury Lane Management Company Limited | Households | 2015 |
| One Day at a Time Community CIC | Health & social care | 2025 |
| One Day Cleaners Limited | Other services | 2012 |
| One Direction Educator Limited | Education | 2014 |
| One Drive Solutions Ltd | Wholesale & retail | 2025 |
| One House of Harts Ltd | Wholesale & retail | 2022 |
| One London Church | Other services | 2015 |
| One Love Healing Limited | Health & social care | 2023 |
| One Love Padel Ltd | Wholesale & retail | 2025 |
| One Managementt Inc. Ltd | Arts & recreation | 2025 |
| One Mart Ltd | Wholesale & retail | 2025 |
| One Mercer Retail Ltd | Hospitality | 2026 |