| Oaklands Assessment Services Limited | Education | 2000 |
| Oaklands Commercial Ltd. | Arts & recreation | 2014 |
| Oaklands Management (ST Albans) Freehold Limited | Administrative & support | 2006 |
| Oakline Management Limited | Households | 2017 |
| Oaks Tech (UK) Co, Ltd | Professional & scientific | 2018 |
| Oaktree Systems Limited | Information & communication | 2013 |
| Oasis Bookkeeping & Payroll Services Ltd | Administrative & support | 2018 |
| Oasis Mini Market (ST Albans) Limited | Wholesale & retail | 2022 |
| Oasis Recruitment Limited | Administrative & support | 1995 |
| Obex Print Limited | Real estate | 1991 |
| Object.View Limited | Information & communication | 2000 |
| Obliq Ltd | Information & communication | 2026 |
| Obliquity Group Limited | Professional & scientific | 2016 |
| OC BYW Limited | Real estate | 2021 |
| OC Living Limited | Real estate | 2021 |
| Ocean Drive Overseas Limited | Administrative & support | 2010 |
| Ocean Post Production Limited | Administrative & support | 2012 |
| Ocean Prime Industries Ltd | Information & communication | 2022 |
| Ocean Star (Fish & Chips) Ltd | Hospitality | 2021 |
| Ocorla Limited | Professional & scientific | 2026 |
| Octa Sports Ltd | Professional & scientific | 2026 |
| October Design Limited | Professional & scientific | 2007 |
| October Film Studios Limited | Information & communication | 2017 |
| Octopus Ventures SPV I LLP | | 2021 |
| Octopus Ventures SPV II LLP | | 2021 |
| Ocular Magnets Ltd | Manufacturing | 2023 |
| Oddarts Media Limited | Professional & scientific | 2024 |
| Oddball Talent Management Ltd | Professional & scientific | 2025 |
| Oddly Brilliant Limited | Professional & scientific | 2023 |
| Odessa Consulting Ltd | Professional & scientific | 2017 |
| Odus Limited | Other services | 2014 |
| Odyzzey Shipping Ltd | Transport & logistics | 2025 |
| OEA Global Limited | Wholesale & retail | 2016 |
| Oeshi Capital Ltd | Financial services | 2021 |
| OF Many Minds Ltd | Information & communication | 2023 |
| Office Gynaecology Limited | Health & social care | 2022 |
| Offspring Services Ltd | Professional & scientific | 2025 |
| Ogier Electronics Limited | Information & communication | 1993 |
| OGM Hair Ltd | Other services | 2020 |
| Ogmax Limited | Information & communication | 2011 |
| OH Online Ltd | Wholesale & retail | 2023 |
| OHM Trading Ltd | Wholesale & retail | 2020 |
| Ohsocutekids Ltd | Wholesale & retail | 2024 |
| OJ Poole Electrical Services Limited | Construction | 2009 |
| OJT Construction Limited | Construction | 2026 |
| Oklahoma! Screen Productions Limited | Information & communication | 1998 |
| Olam Global Ltd | Professional & scientific | 2019 |
| Olam Security Group Ltd | Professional & scientific | 2019 |
| Old Albanian Rugby Football Club | Arts & recreation | 2016 |
| Old Albanian Sports Association Ltd | Arts & recreation | 2004 |
| Old Brompton Developments Ltd | Real estate | 2019 |
| Old Garden Court Management Limited(The) | Real estate | 1982 |
| Old London Road Garage (ST Albans) Limited | Wholesale & retail | 2008 |
| Old London Road Garage Limited | Administrative & support | 2008 |
| Old Paparazzi Ltd | Hospitality | 2020 |
| Old School Management Company Limited(The) | Households | 1977 |
| Oleka Limited | Transport & logistics | 2023 |
| Olino Creations Ltd | Arts & recreation | 2021 |
| Olisson Construction Ltd | Construction | 2014 |
| Olive & Costa Ltd | Professional & scientific | 2021 |
| Olive Tree Gardens (OTG) Ltd | Other services | 2020 |
| Oliver & Akers Ltd | Real estate | 2014 |
| Oliver IR Ltd | Professional & scientific | 2020 |
| Olivers Consultancy Limited | Professional & scientific | 2018 |
| Olivers Equestrian Limited | Agriculture & fishing | 2022 |
| Olivia Tetlow Limited | Arts & recreation | 2023 |
| Olix Computing Limited | Information & communication | 2024 |
| Oljag Ltd | Education | 2019 |
| Ollie Glaze Bakery Ltd | Professional & scientific | 2026 |
| Oloverdaplace Ltd | Professional & scientific | 2015 |
| Olympos Loukouma Ltd | Hospitality | 2025 |
| Olympus Mobile Tyre Fitting ST Albans Ltd | Wholesale & retail | 2023 |
| Omar Zafar Ltd | Transport & logistics | 2025 |
| Omega Project Electrical Ltd | Construction | 2024 |
| Omicron Solutions Limited | Information & communication | 1993 |
| OMM App Limited | Information & communication | 2022 |
| Omni Basics Limited | Wholesale & retail | 2024 |
| Omni Consultancy Limited | Information & communication | 2015 |
| Omni Lifetime Planning Limited | Financial services | 2013 |
| Omniplex Learning Bidco Limited | Financial services | 2025 |
| Omniplex Learning Topco Limited | Financial services | 2025 |
| Omnipresent Systems Limited | Information & communication | 2008 |
| Omnuron Ltd | Information & communication | 2026 |
| Omsins Express Ltd | Hospitality | 2025 |
| Omvac Ltd | Professional & scientific | 2025 |
| ON a Mission Grips Limited | Information & communication | 2013 |
| ON TAP Future Ltd | Professional & scientific | 2016 |
| ON the Chain Ltd | Information & communication | 2022 |
| One Call Building Services (Herts) Ltd | Construction | 2013 |
| One Home Interiors Ltd | Professional & scientific | 2021 |
| One in a Million Solutions Ltd | Information & communication | 2011 |
| One Medical Clinics Limited | Health & social care | 2021 |
| Oneiro Travel Ltd | Administrative & support | 2024 |
| Onepoint IT Ltd | Professional & scientific | 2024 |
| Onesales Ltd | Information & communication | 2025 |
| Online Kitchen Store Ltd | Manufacturing | 1997 |
| Online Seating Ltd | Manufacturing | 2025 |
| Onobrad Ltd | Extraterritorial | 2020 |
| Onon Holdings Ltd | Financial services | 2024 |
| ONS Global Ltd | Construction | 2023 |