| ON Time Bride Limited | Information & communication | 2025 |
| ON Top of the Mountain Ltd | Education | 2005 |
| One Ellen Street Ltd | Real estate | 2018 |
| One Grand Avenue Hove RTM Company Limited | Extraterritorial | 2025 |
| One Night Stand Events Limited | Administrative & support | 2026 |
| One Point Creative Ltd. | Information & communication | 2015 |
| One Root Mortgages Limited | Financial services | 2025 |
| One ST. Catherine's Terrace Hove Limited | Real estate | 1982 |
| Onedata Limited | Information & communication | 1989 |
| Oneidme Limited | Information & communication | 2013 |
| Onemed Ltd | Health & social care | 2017 |
| Onemed Properties Limited | Real estate | 2015 |
| Oneofakind Awakenings Limited | Education | 2022 |
| Onetennis Ltd | Arts & recreation | 2026 |
| ONI Palokaj Ltd | Manufacturing | 2023 |
| Online Orthodontic Academy Limited | Education | 2024 |
| Onlinetraining Limited | Education | 2008 |
| Onme Platforms Ltd | Wholesale & retail | 2025 |
| Onoa NJ Limited | Arts & recreation | 2025 |
| Onpoint Roofing and Guttering Ltd | Construction | 2025 |
| Onthenose Productions Ltd | Arts & recreation | 2024 |
| Onuralp Ltd | Wholesale & retail | 2019 |
| Ookit Ltd | Hospitality | 2024 |
| Oomska Media Limited | Information & communication | 2015 |
| OPC Construction Limited | Construction | 2019 |
| Opendror Ltd | Professional & scientific | 2025 |
| OPS Squad Limited | Administrative & support | 2014 |
| Opsl Ltd | Health & social care | 2023 |
| Optimal Structural Engineers Ltd | Professional & scientific | 2011 |
| Optimum Landscapes Southern Ltd | Administrative & support | 2024 |
| Optimum Maintenance & Cleaning Ltd | Administrative & support | 2023 |
| Optimum Plumbing & Heating (Sussex) Ltd | Construction | 2022 |
| Optimum Property Services Sussex Limited | Administrative & support | 2017 |
| Oral Hygiene Limited | Health & social care | 2013 |
| Orama Productions Ltd | Information & communication | 2012 |
| Orange Movement Ltd | Arts & recreation | 2022 |
| ORB Illustration Limited | Arts & recreation | 2008 |
| Orbyss Ltd | Information & communication | 2026 |
| Orchard Gate Homes Limited | Construction | 2020 |
| Orchard Gate New Homes Limited | Construction | 2021 |
| Orchard Holdings (Hove) Limited | Real estate | 2022 |
| Orchidea Design Ltd | Manufacturing | 2020 |
| Oregano Consultants Ltd | Information & communication | 2015 |
| Orelia Limited | Wholesale & retail | 2005 |
| Organic Earth Market Limited | Wholesale & retail | 2019 |
| Organic Food Ltd | Wholesale & retail | 2021 |
| Organic Frenchie Ltd | Arts & recreation | 2026 |
| Organic Monkey Limited | Manufacturing | 2009 |
| Oriental Massage & Spa Ltd | Other services | 2025 |
| Origin Consultancy Limited | Information & communication | 1993 |
| Origin Working Capital Solutions Ltd | Financial services | 2024 |
| Origins Classic Cars Limited | Wholesale & retail | 2017 |
| Orion Investigations & Intelligence Ltd | Administrative & support | 2014 |
| Orion Investigations Limited | Administrative & support | 2018 |
| ORL Solutions Limited | Health & social care | 2020 |
| Orphans International | Hospitality | 2012 |
| Orsa Limited | Hospitality | 2019 |
| Orzo Blue Limited | Wholesale & retail | 2023 |
| Osar OLA Retail Ltd | Wholesale & retail | 2025 |
| Osborne Mansions (Hove) Limited | Households | 2017 |
| Oscar Leonards Property Management Company Limited | Extraterritorial | 2014 |
| Osmond Road Management Limited | Households | 2011 |
| OSO Insights Ltd | Information & communication | 2022 |
| Osoli Ltd | Information & communication | 2013 |
| Ostell Associates Limited | Administrative & support | 2011 |
| Ostell Holdings Limited | Financial services | 2022 |
| Osteosian Limited | Health & social care | 2025 |
| Othermind Collective Ltd | Wholesale & retail | 2025 |
| Otherton Solar Limited | Energy | 2015 |
| Otis Films Ltd | Information & communication | 2019 |
| Ottewell Estates Limited | Households | 2022 |
| Ouoficelinoers Limited | Information & communication | 2025 |
| Outdoor FUN Stuff Ltd | Arts & recreation | 2019 |
| Outlaw Marine Limited | Arts & recreation | 2010 |
| Outlook Capital Ltd | Real estate | 2018 |
| Outwood Solar Limited | Energy | 2012 |
| Ovensparkle Limited | Administrative & support | 2020 |
| Ovensupport Limited | Other services | 2017 |
| Over the Rainbow Playschool Ltd | Health & social care | 2024 |
| Own . ACT Ltd | Professional & scientific | 2013 |
| OX Generation Limited | Other services | 2009 |
| OZ Happy Planet Limited | Wholesale & retail | 2023 |
| OZC Group Ltd | Construction | 2024 |
| Ozer Group Ltd | Hospitality | 2016 |
| Ozland Group Limited | Other services | 2017 |
| O’Kinnell Ltd | Arts & recreation | 2023 |
| O’Kinnell Sports Ltd | Arts & recreation | 2023 |
| P B Aerials Ltd | Information & communication | 2007 |
| P C Community Care Limited | Professional & scientific | 2006 |
| P P F Food Ltd | Hospitality | 2020 |
| P R Homes Direct Holdings Ltd | Financial services | 2025 |
| P S Group (Southern) Ltd | Construction | 2023 |
| P W Builders Limited | Construction | 2020 |
| P&D Healthcare Ltd | Health & social care | 2021 |
| P&P Chiswick Limited | Health & social care | 2019 |
| P&P Dent Holdings Limited | Health & social care | 2018 |
| P&P Golders Green Limited | Health & social care | 2023 |
| P&P Property Enterprise Limited | Real estate | 2020 |
| P&S Fixings Ltd | Construction | 2025 |
| P&S Formwork Limited | Construction | 2018 |