| Number Fifteen Church Street Limited | Other services | 2005 |
| O & N Limited | Real estate | 2002 |
| Oakland Interim Management Limited | Professional & scientific | 2002 |
| Oaklands Association Limited | Other services | 2002 |
| Oaklands Commercial Management Ltd | Professional & scientific | 2015 |
| Oaklands Contracts Limited | Construction | 2017 |
| Oaklands Hair Ltd | Other services | 2014 |
| Oaklands House Residents Limited | Households | 1999 |
| Oaklands Neurology Ltd | Health & social care | 2026 |
| Oaklands Plumbing Company Ltd | Construction | 2026 |
| Oaklands Property Services Limited | Real estate | 2008 |
| Oaklands Residents Association (Welwyn) Limited | Households | 1980 |
| Oaklea Wood Limited | Real estate | 2010 |
| Oakwood Dermatology LLP | | 2011 |
| OBR Consulting Ltd | Other services | 2023 |
| Obsidian Structures Limited | Professional & scientific | 2024 |
| OCC Bloodstock Ltd | Agriculture & fishing | 2022 |
| Oeuniversal Limited | Arts & recreation | 2023 |
| Old Elizabethans (Barnet) Playing Fields Limited(The) | Arts & recreation | 1950 |
| Old House Estate Residents Association (Tewin) Limited (the) | Households | 1967 |
| Old Welwyn Auto Centre Ltd | Other services | 2025 |
| Old Welwyn Construction Ltd | Construction | 2026 |
| Old Welwyn MOT Ltd | Extraterritorial | 2019 |
| Omani Investments Limited | Real estate | 2010 |
| Omniac Investments Limited | Financial services | 2020 |
| Omniac Properties Limited | Real estate | 2018 |
| One Reality Ltd | Information & communication | 2026 |
| One Roof Social Ltd | Professional & scientific | 2015 |
| Opallocal Property Management Limited | Households | 2002 |
| Open Green Ltd | Information & communication | 2024 |
| Orange Griffin Ltd | Information & communication | 2013 |
| Original Cultures | Information & communication | 2009 |
| Orion Marine Solutions Ltd | Information & communication | 2017 |
| Osirah Ltd | Wholesale & retail | 2026 |
| OSK Support Limited | Administrative & support | 2022 |
| OUR Kushi Ltd | Information & communication | 2011 |
| Outtrader Ltd | Information & communication | 2020 |
| OWR Solutions Ltd | Professional & scientific | 2025 |
| P&M Restaurants Limited | Hospitality | 2023 |
| PAL Information & Strategic Solutions Limited | Information & communication | 1998 |
| Palm Air Cargo Limited | Transport & logistics | 2018 |
| Palmadene Limited | Construction | 2001 |
| Parallel HR Ltd | Administrative & support | 2012 |
| Park Lane Family Holdings Limited | Financial services | 2021 |
| Parkside (Welwyn) Management Company Limited | Households | 1984 |
| Parry Advisory Limited | Professional & scientific | 2019 |
| Patentest UK Limited | Professional & scientific | 2017 |
| Patwal Investments Ltd | Real estate | 2024 |
| Paul Smith Lettings Limited | Real estate | 2023 |
| Paulmar Trading Limited | Other services | 2023 |
| PC Assurance Ltd | Real estate | 2021 |
| Pearl East Automation Ltd | Manufacturing | 2018 |
| Peglers Wholesale UK Ltd | Wholesale & retail | 2021 |
| Perfect Solutions Global Ltd | Information & communication | 2004 |
| Performance K9 Ltd | Other services | 2025 |
| Philip Morris Property Consultants Limited | Real estate | 2013 |
| Phillip Graham Sports Management Limited | Arts & recreation | 1996 |
| Philpin Ltd | Construction | 2017 |
| Phygicool Labs Ltd | Professional & scientific | 2013 |
| Pierre Monribot Limited | Administrative & support | 2006 |
| Pinerise Limited | Households | 1998 |
| Pioneer Spirits Ltd | Wholesale & retail | 2018 |
| Planet Weddings Limited | Administrative & support | 2011 |
| PMR Management Limited | Professional & scientific | 2005 |
| POD Sports Limited | Arts & recreation | 2019 |
| Podcast Discovery Limited | Administrative & support | 2022 |
| Portal IT Services Limited | Information & communication | 2025 |
| Pottersheath Associates Ltd | Information & communication | 2010 |
| Power Technical Library LLP | | 2019 |
| Powerkut Limited | Professional & scientific | 2016 |
| PPW Investments Limited | Other services | 2021 |
| PPW Property Investments Limited | Real estate | 2020 |
| Prestige Financial Planning Limited | Professional & scientific | 2011 |
| Prestige Trim Ltd | Wholesale & retail | 2025 |
| Price's Placements Ltd | Administrative & support | 2023 |
| Prime Lift Interiors Ltd | Professional & scientific | 2025 |
| Primett Limited | Real estate | 2001 |
| Priors Environmental Limited | Agriculture & fishing | 2009 |
| Private SW Ltd | Health & social care | 2024 |
| Prize Time Limited | Arts & recreation | 2023 |
| Proclense Facilities Management Limited | Other services | 2015 |
| Proclense Valeting Services Limited | Other services | 2009 |
| Prodach Ltd | Construction | 2017 |
| Product Management AS a Service Ltd | Information & communication | 2026 |
| Profinance Ltd | Professional & scientific | 2022 |
| Profusion Welwyn Limited | Wholesale & retail | 2010 |
| Progrowth Consulting Ltd | Professional & scientific | 2023 |
| Property & Co Partners Ltd | Real estate | 2025 |
| Proshare Limited | Real estate | 2023 |
| Provide-Ed Ltd | Education | 2020 |
| Public Sector Consulting Ltd | Administrative & support | 2018 |
| Pyttipanna Ltd | Professional & scientific | 2021 |
| Q-Dots Ltd | Professional & scientific | 2026 |
| Quadrant Pharmacies Ltd | Wholesale & retail | 2003 |
| Quant Solutions Ltd | Information & communication | 2013 |
| Quantum Advice Limited | Professional & scientific | 2026 |
| Quayc.Com Limited | Administrative & support | 2004 |
| R Hoad Plumbing and Heating Ltd | Construction | 2014 |
| Race Bred Ltd | Information & communication | 2017 |
| Randalls Crane Hire Ltd | Administrative & support | 2016 |