| Oakwood Drive Harpenden Management Company Ltd | Real estate | 1999 |
| Oaltaan Holding Limited | Financial services | 2024 |
| OB Assets Ltd | Real estate | 2017 |
| Obcodi Ltd | Administrative & support | 2014 |
| Obsidian Investment Solutions Ltd. | Financial services | 2023 |
| Ocasta Studios Limited | Information & communication | 2011 |
| Oceanwrap Limited | Professional & scientific | 2012 |
| Octavian Consultants Limited | Professional & scientific | 2021 |
| Ojas Sauna Ltd | Other services | 2024 |
| Ojcamazonfba Limited | Wholesale & retail | 2023 |
| Okes Healthcare Limited | Health & social care | 2021 |
| Okolieboxing Ltd | Arts & recreation | 2016 |
| Oksoko Ltd | Real estate | 2023 |
| OLA Oshin Limited | Real estate | 2022 |
| Old Dairy Management (WGC) Limited | Households | 2016 |
| Old Glebe Barns Limited | Wholesale & retail | 2015 |
| Old Post House Management Company Limited | Professional & scientific | 2014 |
| Old School Animal Care Ltd | Professional & scientific | 2017 |
| Oleum Energy Ltd | Administrative & support | 2021 |
| Olimas Space Ltd | Real estate | 2020 |
| Olivana Limited | Information & communication | 2017 |
| Oliwkowo Ltd | Real estate | 2026 |
| Omni Home Design Ltd | Wholesale & retail | 2010 |
| ON Air RF Limited | Professional & scientific | 2005 |
| One 2 One Tax and Accountancy Limited | Professional & scientific | 2012 |
| One Sky Consulting Ltd | Professional & scientific | 2024 |
| One TEN LDN Ltd | Information & communication | 2025 |
| Onejp Consulting Ltd | Professional & scientific | 2018 |
| Onejp Holding Ltd | Professional & scientific | 2018 |
| Onlinewills Ltd | Professional & scientific | 2026 |
| Onside Football Management Ltd | Professional & scientific | 2017 |
| Onyuma Limited | Professional & scientific | 2023 |
| OPE47 Ltd | Professional & scientific | 2022 |
| Open Hairdressing Harpenden Limited | Administrative & support | 2017 |
| Opera Schmopera Ltd | Education | 2016 |
| Operyx Ltd | Information & communication | 2026 |
| Optima Computer Consultants Ltd | Information & communication | 2009 |
| Optimal Care Group Ltd | Health & social care | 2025 |
| Optimal Franchise Ltd | Wholesale & retail | 2023 |
| Oracle Social Work Ltd | Health & social care | 2026 |
| Orbbis Ltd | Administrative & support | 2022 |
| Orden Kelly Ltd | Real estate | 2016 |
| Oriental Aroma Limited | Hospitality | 2017 |
| Orion AI Engineering Ltd | Education | 2022 |
| Orly Renewables Ltd | Professional & scientific | 2026 |
| Orton Place Residents Management Co. Limited | Households | 2023 |
| Orwell Paving & Landscapes Limited | Administrative & support | 2016 |
| Oshin Pathfinders Limited | Other services | 2024 |
| Osprey Close (Harpenden) Management Company Limited | Households | 2014 |
| Ostrich Grips Limited | Information & communication | 2014 |
| Ottasilver Ltd | Wholesale & retail | 2018 |
| OUI of Harpenden Ltd | Wholesale & retail | 2022 |
| Outpace World Ltd | Professional & scientific | 2025 |
| Overdrive Enterprises Limited | Real estate | 2002 |
| Overstone 9801 Limited | Professional & scientific | 2026 |
| OWL Strategy Ltd | Professional & scientific | 2026 |
| Oxen Court Limited | Real estate | 2012 |
| Oxley Timber Products Limited | Professional & scientific | 2012 |
| Ozcan Sourcing UK Ltd | Wholesale & retail | 2019 |
| Ozdemir Construction Ltd | Construction | 2020 |
| Ozee Agriculture and Landscape Gardening Ltd | Professional & scientific | 2020 |
| Ozgur Car Body Repair Ltd | Wholesale & retail | 2021 |
| Ozkaya Engineering and Consultancy Ltd | Information & communication | 2021 |
| P Campbell Consulting Ltd | Professional & scientific | 2021 |
| P Ellis Management Company Ltd | Administrative & support | 2020 |
| P J Kirkpatrick Design and Build Limited | Construction | 2015 |
| P&G Estates Ltd | Real estate | 2024 |
| P&S Advisory Ltd | Administrative & support | 2023 |
| Pacchetto Limited | Hospitality | 2024 |
| Pacnov Chartering and Logistics Limited | Transport & logistics | 2023 |
| Paddock View Developments Limited | Construction | 2016 |
| Padel Operations UK Limited | Arts & recreation | 2025 |
| Paeoniae Limited | Real estate | 2014 |
| Page 8 Productions Limited | Information & communication | 2018 |
| Paglets Ltd | Real estate | 2021 |
| Paint IT Orange Ltd | Wholesale & retail | 2016 |
| Pakcsco Ltd | Health & social care | 2012 |
| Palmer Public Relations Limited | Professional & scientific | 2005 |
| Palmer-Stoves Associates Limited | Professional & scientific | 2006 |
| Palmers Farm Development Ltd | Construction | 2026 |
| Panamon Ltd | Wholesale & retail | 2023 |
| Para Consulting Ltd | Professional & scientific | 2024 |
| Paradox House Ltd | Information & communication | 2019 |
| Park Avenue (29) Limited | Real estate | 2015 |
| Park Securities Limited | Financial services | 1994 |
| Parkguard Limited | Administrative & support | 2007 |
| Parm Entertainment Limited | Information & communication | 2025 |
| PARSE#7 Consultants Limited | Professional & scientific | 2021 |
| Parties, Tents and Events Ltd | Arts & recreation | 2022 |
| Partners in Change Limited | Professional & scientific | 1991 |
| Partorenewables Ltd | Information & communication | 2024 |
| Partsmartuk Ltd | Wholesale & retail | 2026 |
| Pasaan Limited | Information & communication | 2024 |
| Passing Presence Ltd | Other services | 2024 |
| Pathfinder Real Estate Limited | Real estate | 2017 |
| Pathways Dementia Support Ltd | Education | 2025 |
| Patmayan COM Ltd | Wholesale & retail | 2022 |
| Paul 1 Smith Carpentry Limited | Construction | 2017 |
| Paul Richardson T/As Claims Solutions Ltd | Financial services | 2015 |
| Pawd Drinks Limited | Manufacturing | 2023 |