| Olubu Ltd | Wholesale & retail | 2016 |
| OM Food & Wine Limited | Wholesale & retail | 2022 |
| OM Nilakantha Ltd | Real estate | 2025 |
| OM SAI Shree Co. Limited | Wholesale & retail | 2021 |
| OMA Smiles Limited | Health & social care | 2023 |
| Omat International Limited | Transport & logistics | 1985 |
| OMC Properties Ltd | Households | 2026 |
| OMI Leisure Ltd | Hospitality | 2017 |
| Omni Dental Ltd | Health & social care | 2024 |
| Omrah Limited | Professional & scientific | 2016 |
| ON Task Developments Ltd | Construction | 2024 |
| ON the Rocks Events Ltd | Hospitality | 2025 |
| ON the SLY Holdings Limited | Financial services | 2022 |
| ON the SLY Ltd | Information & communication | 2003 |
| ON the SLY Properties Ltd | Real estate | 2022 |
| On-Target Training Network Limited | Education | 2006 |
| Onami Consulting Ltd | Professional & scientific | 2022 |
| One Capital RE Limited | Financial services | 2014 |
| One Family Hub Limited | Health & social care | 2026 |
| One Forty Four Projects Ltd | Information & communication | 2025 |
| One Investments Global Limited | Real estate | 2013 |
| One Mind Clinic Ltd | Health & social care | 2018 |
| One Stop Wedding Shop Ltd | Administrative & support | 2009 |
| Onesimus Ltd | Extraterritorial | 1997 |
| Online Ticket Seller Ltd | Administrative & support | 2015 |
| Onsaran Limited | Financial services | 2025 |
| Onyx Property Ltd | Real estate | 2014 |
| Ootama Limited | Extraterritorial | 2021 |
| OP Dental Limited | Health & social care | 2013 |
| OPA2 Ltd | Hospitality | 2026 |
| Opal Insight Limited | Professional & scientific | 2012 |
| Opekta Limited | Real estate | 2021 |
| Open Digital Cooperative Ltd | Information & communication | 2022 |
| Open Mutual Ltd. | Information & communication | 2000 |
| Operation Save Jamaica (UK) | Other services | 2018 |
| Opportune Dental Limited | Health & social care | 2021 |
| Opsis Property LLP | | 2010 |
| Opsyon Cauri Limited | Financial services | 2014 |
| Optics Investments Ltd | Real estate | 2022 |
| Optimise Ecom Ltd | Information & communication | 2018 |
| Optimised Services Provider Limited | Professional & scientific | 2021 |
| Optimum Financial Services Ltd | Financial services | 2023 |
| Optimum Microsystems Ltd. | Professional & scientific | 2013 |
| Opulent Elements Limited | Wholesale & retail | 2025 |
| Orange Lizard Ltd | Administrative & support | 2016 |
| Orange Room Music Limited | Information & communication | 2013 |
| Orbit Performance and Styling Limited | Wholesale & retail | 2025 |
| Orbital Roofing & Building Specialists Limited | Construction | 2001 |
| Orchard House Rest Home Limited | Hospitality | 2001 |
| Orchard Osteopathy Limited | Health & social care | 2024 |
| Orchid Homes (Construction) Limited | Construction | 2019 |
| ORE G Ltd | Real estate | 2025 |
| Oriel Consultancy Ltd | Professional & scientific | 2024 |
| Original Gravity Media Ltd | Information & communication | 2018 |
| ORK Investments Limited | Real estate | 2024 |
| Orphic Resonance Ltd | Arts & recreation | 2024 |
| Oryx Property Sussex Limited | Real estate | 2023 |
| Oryzeon Limited | Information & communication | 2025 |
| Osborne Berry Installations Limited | Construction | 2002 |
| Osborne Plastering Limited | Construction | 2012 |
| Osborne Vans Ltd | Wholesale & retail | 2026 |
| Osborne Videos Ltd | Information & communication | 2024 |
| Osbourne House (Eastbourne) Management Company Limited | Households | 2017 |
| Oscar Lewenstein Productions Limited | Administrative & support | 1954 |
| Oscars Eastbourne Ltd | Other services | 2009 |
| Osco Homes Limited | Construction | 2015 |
| Osiry Ltd | Information & communication | 2021 |
| Osraya Limited | Other services | 2026 |
| Osteriahove Limited | Hospitality | 2023 |
| Ostraca Limited | Construction | 2019 |
| Otaara Limited | Professional & scientific | 2017 |
| Otopia Software Ltd | Information & communication | 2012 |
| Otro Events Limited | Other services | 2018 |
| Ottoh Limited | Health & social care | 2025 |
| Otus Designs Limited | Wholesale & retail | 2013 |
| OUR Little Markets CIC | Wholesale & retail | 2019 |
| OUR Neighbourhood CIC | Education | 2021 |
| OUR Place Restaurant Limited | Hospitality | 2011 |
| Outoftheboxthinking.Co Ltd | Education | 2021 |
| Outsideology Limited | Administrative & support | 2015 |
| Overseas Travel Limited | Real estate | 2023 |
| Ovingdean Estates Limited | Construction | 2014 |
| OW East Sussex Ltd | Administrative & support | 2016 |
| Owen Groundworks Limited | Construction | 2013 |
| Owen-Roberts Property Investments Limited | Real estate | 2020 |
| OWL Structural Limited | Professional & scientific | 2015 |
| Owlets Nursery Limited | Education | 2017 |
| Own a Home Community Interest Company | Households | 2019 |
| Owners Consortium Limited | Real estate | 2002 |
| Ownership Transference Ltd | Financial services | 2024 |
| Oxbo Property Ltd | Real estate | 2020 |
| Oxendean Gardens (Willingdon) Limited | Real estate | 2004 |
| Oxendean Limited | Administrative & support | 2013 |
| Oxford Bone Research Ltd | Health & social care | 2020 |
| Oxford Computer Resources Limited | Administrative & support | 2017 |
| Oxford Farmhouse CIC | Wholesale & retail | 2020 |
| Oxfordian Archive Limited | Administrative & support | 2025 |
| Oxted T and R Limited | Construction | 2025 |
| OZ Tots Limited | Education | 2020 |
| Ozler FN Ltd | Hospitality | 2015 |