| OBN Services Limited | Administrative & support | 2021 |
| Obsidian INK Taunton Ltd | Arts & recreation | 2025 |
| Obvera Ltd | Professional & scientific | 2025 |
| Occupation Navigation Ltd | Other services | 2019 |
| Occuteach Ltd | Education | 2023 |
| Ocean Blue Fish N Meat Ltd | Wholesale & retail | 2025 |
| Ocean Images (UK) Limited | Professional & scientific | 1992 |
| Octagon Management Company Limited | Households | 1986 |
| Octane Distribution Limited | Wholesale & retail | 2001 |
| Octaneologists Limited | Wholesale & retail | 2023 |
| OD Consultants Ltd | Professional & scientific | 2026 |
| Odds on Property Ltd | Real estate | 2026 |
| Odessa International Brokerage Limited | Public administration | 2025 |
| OEP Ltd | Wholesale & retail | 2025 |
| OET Access Ltd | Health & social care | 2022 |
| OFF Road on TAP Limited | Wholesale & retail | 2023 |
| OFF the Levels Inn’S Limited | Hospitality | 2022 |
| OFF the Pole Ltd | Administrative & support | 2016 |
| Offerlink Limited | Real estate | 1999 |
| Office Revival Limited | Administrative & support | 2009 |
| Offshore Doors Ltd | Manufacturing | 2022 |
| Offshore Positioning Solutions Limited | Mining & quarrying | 2009 |
| OG Carbon Ltd | Professional & scientific | 2022 |
| OGK Property Holdings Limited | Real estate | 2024 |
| Ohana Projects Holdings Ltd | Financial services | 2023 |
| Oils 2 U Ltd | Wholesale & retail | 2019 |
| OJR Aircraft Maintenance Limited | Manufacturing | 2019 |
| Okwuosa Services Ltd | Health & social care | 2022 |
| Old Cleeve House Management Limited | Households | 2007 |
| Old Coastguard Holiday Park Limited | Hospitality | 2014 |
| Old Dock Investments Limited | Financial services | 2018 |
| Old Hall 2022 Limited | Real estate | 2014 |
| Old Market Walk (Sturminster Newton) Management Company Limited | Households | 2005 |
| Old Oak Forest, Garden & Home Services and Trading Ltd | Agriculture & fishing | 2021 |
| Old Sarum House Yeovil Limited | Households | 2007 |
| Old Trade Winds Ltd | Wholesale & retail | 2024 |
| Olimi Hair & Beauty Gallery Limited | Other services | 2006 |
| Olive HUT Enterprises Limited | Real estate | 2023 |
| Olive Star Group Ltd | Wholesale & retail | 2024 |
| Olive Tree Foundation Limited | Professional & scientific | 2014 |
| Olive Tree Property Investments Limited | Financial services | 2022 |
| Olive Tree Property Rentals LLP | | 2022 |
| Oliver Nutland Contracting Limited | Agriculture & fishing | 2003 |
| Oliver Twiby Audio Support Limited | Other services | 2014 |
| Olivia Counts Limited | Professional & scientific | 2023 |
| Olympus IB Limited | Financial services | 2025 |
| OM Studios LLP | | 2020 |
| Omikse. LP | | 2016 |
| Ommen Properties Ltd | Real estate | 2022 |
| Omnilocum Ltd | Health & social care | 2017 |
| ON TAP Audio Ltd | Information & communication | 2025 |
| ON TAP Plumbing (South West) Limited | Construction | 2009 |
| ON the Box Limited | Information & communication | 2010 |
| ON the Level Limited | Information & communication | 1999 |
| ON the Levels Building Engineers Ltd | Extraterritorial | 2024 |
| ON Your Bike (Recycle) Ltd | Manufacturing | 2011 |
| ON Your Bike Bridgwater Ltd | Wholesale & retail | 2018 |
| ON Your Bike Taunton Ltd | Wholesale & retail | 2018 |
| Onbezorgd Ondernemen LLP | | 2015 |
| One CUT Tree Services Ltd | Administrative & support | 2025 |
| One Love Home Care Service Limited | Health & social care | 2022 |
| One Plus One Taunton Ltd | Professional & scientific | 2007 |
| One SIX Engineering Limited | Manufacturing | 2025 |
| One Step Recruitment Ltd | Administrative & support | 2012 |
| One to Love Limited | Financial services | 2026 |
| One Twenty Two (Taunton) Management Limited | Households | 2008 |
| One Way Road Limited | Professional & scientific | 2017 |
| Onebow & Co. Ltd | Wholesale & retail | 2025 |
| Online Digital Shopping Services Ltd | Wholesale & retail | 2023 |
| Online Stylist Limited | Wholesale & retail | 2006 |
| Open Charged Ltd | Manufacturing | 2023 |
| Open Mind Technologies UK Limited | Information & communication | 2000 |
| Open Payment Network Limited | Information & communication | 2015 |
| Open Suitcase Holidays Ltd | Transport & logistics | 2024 |
| Open Vision Finance Limited | Financial services | 2023 |
| Open Vision Wealth Limited | Financial services | 2025 |
| Opera House NO.2 Limited | Households | 2010 |
| OPN Global Limited | Information & communication | 2020 |
| OPT Financial Limited | Financial services | 2017 |
| OPT Group Limited | Real estate | 2018 |
| Optev Ltd | Information & communication | 2026 |
| Optimaal Administraties LLP | | 2017 |
| Optimised Logic Limited | Information & communication | 2010 |
| Optimum Acoustics Ltd | Manufacturing | 2024 |
| Optimum Performance Riding Saddles Ltd | Manufacturing | 2023 |
| Optimus Education Limited | Education | 2006 |
| Opulent (SW) Limited | Real estate | 2024 |
| Opus Management and Consultancy Limited | Professional & scientific | 2012 |
| Opus Paraplanning Ltd | Financial services | 2025 |
| Opusserve Ltd | Information & communication | 2025 |
| Oracle Builders Ltd | Construction | 2020 |
| Orange Ocelot Ltd | Wholesale & retail | 2023 |
| Orangemarble Ltd | Administrative & support | 2018 |
| Orangepeel Lighting Design Ltd | Professional & scientific | 2018 |
| Orchard Barton Ltd | Health & social care | 2024 |
| Orchard Carpentry Ltd | Construction | 2026 |
| Orchard Court Holdings Limited | Health & social care | 2017 |
| Orchard Farm (Somerset) Ltd | Agriculture & fishing | 2017 |
| Orchard Financial Consulting Limited | Professional & scientific | 2024 |
| Orchard Grove Management Company (Taunton) Limited | Households | 2026 |