| Oj's Groundworks and Construction Limited | Construction | 2018 |
| Oj's Holdings Ltd | Financial services | 2015 |
| OJC Autos Limited | Wholesale & retail | 2025 |
| OJC Ebikes Limited | Wholesale & retail | 2025 |
| OJH Enterprises Portsmouth Ltd | Wholesale & retail | 2026 |
| OJS Estates Management Ltd | Real estate | 2025 |
| OK & Daughters Limited | Health & social care | 2018 |
| Okay YES Please Limited | Professional & scientific | 2022 |
| Okelasimen Ltd | Hospitality | 2021 |
| Olatco Concepts Limited | Health & social care | 2026 |
| Old Harbour Mews Management Company Limited | Hospitality | 2000 |
| Old House Investments Limited | Professional & scientific | 1984 |
| Old Oak Building & Landscaping Limited | Administrative & support | 2023 |
| Old PAN Limited | Hospitality | 2019 |
| Old Portsmouth College Limited | Education | 2024 |
| Old Reservoir Road Limited | Real estate | 2010 |
| Old VIC Limited | Hospitality | 2018 |
| Old VIC Property Rentals Limited | Real estate | 2015 |
| Olive Homes (Commercial) Ltd | Real estate | 2024 |
| Olive Homes (Residential) Ltd | Real estate | 2026 |
| Olive Homes (Southern) Ltd | Real estate | 2020 |
| Olive Homes (Southsea) Ltd | Real estate | 2024 |
| Olivelli Limited | Professional & scientific | 2016 |
| Oliver Haggard Limited | Real estate | 2016 |
| Oliver Nutkins Limited | Manufacturing | 2016 |
| Olivers Wine Agency Limited | Hospitality | 2011 |
| Olivia's Diner Ltd | Hospitality | 2023 |
| Olukazzy Properties and Investments Ltd | Financial services | 2025 |
| OM & Sunny Dream Limited | Wholesale & retail | 2021 |
| Omer Kaya Limited | Real estate | 2025 |
| Ommoto Ltd | Education | 2023 |
| Omnes in Unum Ltd | Wholesale & retail | 2024 |
| Omni Heating Limited | Construction | 2023 |
| Omnia Consulting Ltd | Professional & scientific | 2021 |
| Omnia Environmental Consulting (North) Ltd | Construction | 2015 |
| Omnia Environmental Consulting (South) Ltd | Professional & scientific | 2013 |
| Omnia Environmental Consulting Ltd | Extraterritorial | 2016 |
| Omnia Environmental Holdings Ltd | Professional & scientific | 2016 |
| Omnia EPS Limited | Professional & scientific | 2025 |
| Omnitek Ltd | Information & communication | 2021 |
| OMO Olofa Limited | Health & social care | 2017 |
| ON Call Medical Services Ltd | Health & social care | 2017 |
| ON Point Electrical Services South Limited | Construction | 2019 |
| ON SET Medical Ltd | Health & social care | 2023 |
| ON Site Catering Solutions Ltd | Hospitality | 2022 |
| ON Site Fabrication Southern Limited | Construction | 2018 |
| ON Site Holdings Limited | Financial services | 2019 |
| ON Site Safety Ltd | Other services | 2006 |
| ON Site Services (Southern) Limited | Wholesale & retail | 2013 |
| On-Site Finance Limited | Financial services | 2021 |
| On-Site Integrated Services Limited | Administrative & support | 2026 |
| On-Site Personnel Limited | Administrative & support | 2019 |
| On-Site Recruitment Limited | Administrative & support | 2019 |
| Ondine Advisory Ltd | Professional & scientific | 2026 |
| One Blue Recruitment Ltd | Administrative & support | 2023 |
| One Solution Recruitment Ltd | Administrative & support | 2019 |
| Onefix Recruitment Ltd | Administrative & support | 2012 |
| Onepipe Plumbing Ltd | Construction | 2016 |
| Onesi Ltd | Hospitality | 2025 |
| Onet Technologies UK Limited | Manufacturing | 1993 |
| Onit Trading Limited | Wholesale & retail | 2024 |
| Online Cleaning UK Limited | Administrative & support | 2009 |
| Online Dispute Resolution Squared Limited | Professional & scientific | 2020 |
| Only Social Recruitment Payroll Ltd | Other services | 2010 |
| Only-Fords Limited | Wholesale & retail | 2024 |
| Onstream Productions Limited | Information & communication | 2025 |
| Ontime Accountants Services Limited | Professional & scientific | 2025 |
| Onyx Land Consulting Ltd | Other services | 2018 |
| Oodleco Ltd | Wholesale & retail | 2026 |
| Open Mind Apartments Ltd | Real estate | 2025 |
| Open Mind Capital Ltd | Real estate | 2024 |
| Open Mind Education Ltd | Education | 2024 |
| Open Mind Empire Ltd | Real estate | 2025 |
| Open Mind Holdings Ltd | Real estate | 2024 |
| Open Mind Homes Ltd | Real estate | 2025 |
| Open Mind Leisure Ltd | Real estate | 2024 |
| Open Mind Property Ltd | Real estate | 2020 |
| Open Mind Stay Ltd | Real estate | 2025 |
| Open Mind Suites Ltd | Real estate | 2025 |
| Openedgetechnologies Ltd | Information & communication | 2026 |
| Operation B2L Ltd | Real estate | 2025 |
| Operation SA Ltd | Real estate | 2025 |
| Operation Serviced Accommodation Ltd | Real estate | 2025 |
| Oprah TIP Top Cleaning Services Ltd | Administrative & support | 2024 |
| Opsai Consulting Ltd | Information & communication | 2026 |
| Opta Group Ltd | Information & communication | 2014 |
| Optamor Limited | Administrative & support | 2001 |
| Optimal Health Care Group Ltd | Health & social care | 2025 |
| Optimism Psychotherapy & Supervision Services Ltd | Health & social care | 2024 |
| Optimum Drain Limited | Water & waste | 2023 |
| Option Call Care Services Limited | Administrative & support | 2016 |
| Opulent Transit Solutions Ltd | Transport & logistics | 2024 |
| Opulenza Ltd | Manufacturing | 2023 |
| Opus Gold Limited | Administrative & support | 2010 |
| Opzu Ltd | Professional & scientific | 2025 |
| Orange Art & Design Ltd | Arts & recreation | 2023 |
| Orange Nails & Spa Ltd | Other services | 2025 |
| Orange Solar Systems Ltd | Administrative & support | 2010 |
| Orbility Limited | Transport & logistics | 2018 |
| Orbit Business Solutions Limited | Information & communication | 2003 |