| Oldham Stars Sports Club Ltd | Arts & recreation | 2023 |
| Oldham Sudanese Supplementary School CIC | Education | 2025 |
| Oldham Supported Living Ltd | Health & social care | 2025 |
| Oldham Taxi Hire Limited | Administrative & support | 2024 |
| Oldham Theatre Workshop Friends’ Company Limited | Arts & recreation | 2020 |
| Oldham Tippers Limited | Construction | 2021 |
| Oldham Total Care Ltd | Health & social care | 2023 |
| Oldham Town Football Club Ltd | Arts & recreation | 2026 |
| Oldham Traction Ltd | Transport & logistics | 2022 |
| Oldham Training Academy Limited | Education | 2018 |
| Oldham Transit Centre Ltd | Wholesale & retail | 2025 |
| Oldham Tyres & Exhausts Centre Limited | Wholesale & retail | 2017 |
| Oldham Tyres & Exhausts Limited | Wholesale & retail | 2017 |
| Oldham Tyres Sales Limited | Wholesale & retail | 2017 |
| Oldham Van Centre Limited | Wholesale & retail | 2015 |
| Oldhamcare Limited | Public administration | 2019 |
| Oldhamowlspreschoolnursery Limited | Health & social care | 2025 |
| Oldroyd Clark Consulting Ltd | Professional & scientific | 2025 |
| Olganna Ltd. | Manufacturing | 2012 |
| Oliben Solutions Ltd | Construction | 2022 |
| Olive & ASH Limited | Administrative & support | 2021 |
| Olive Corporation International Limited | Wholesale & retail | 2007 |
| Olive Tree Hospitality Limited | Hospitality | 2020 |
| Olive Villas Management Company Limited | Real estate | 2005 |
| Oliver James Brooks Ltd | Financial services | 2026 |
| Oliver Mitchell Ltd | Administrative & support | 2018 |
| Olivia Gartley Limited | Administrative & support | 2014 |
| Ollie&Jayden Limited | Health & social care | 2022 |
| Olly Landscapes Ltd | Administrative & support | 2022 |
| OLR Assessment Services Ltd | Education | 2018 |
| Oluona Properties Ltd | Real estate | 2023 |
| OLW Limited | Wholesale & retail | 2023 |
| Olympic Construction Limited | Construction | 2009 |
| Olysem Digital Limited | Information & communication | 2026 |
| OM Metal Limited | Manufacturing | 2025 |
| OM Switchgear & Protection Ltd | Manufacturing | 2021 |
| Omar Solutions Ltd | Other services | 2026 |
| Omar's Mini Market Ltd | Wholesale & retail | 2025 |
| Omara Spa Ltd | Other services | 2026 |
| Omd-Gittal Marketing Ltd | Professional & scientific | 2026 |
| Omega Cosmetics Ltd | Professional & scientific | 2015 |
| Omega Energy Consultants Limited | Wholesale & retail | 2024 |
| Omega Taxi Hire Ltd | Administrative & support | 2017 |
| OMI Productions Ltd | Arts & recreation | 2023 |
| Omnex Solutions Ltd | Wholesale & retail | 2025 |
| Omnia Systems Limited | Information & communication | 2007 |
| Omnia Vehicle Services Ltd | Wholesale & retail | 2026 |
| Omnilogistx Ltd | Professional & scientific | 2025 |
| Omno Glass Limited | Manufacturing | 2025 |
| Omonla Global Limited | Health & social care | 2018 |
| Omvexa Ltd | Wholesale & retail | 2025 |
| ON Clad Ltd | Construction | 2026 |
| ON Padel Oldham Ltd | Arts & recreation | 2026 |
| ON Site Steel Services Limited | Construction | 2017 |
| ON the Move (Royton) Limited | Hospitality | 2025 |
| ON the Pulse of Perfumery Limited | Wholesale & retail | 2011 |
| ON the Wall Metering Ltd | Energy | 2025 |
| Once Upon a Beauty Limited | Other services | 2025 |
| One Eleven Apparel Ltd | Wholesale & retail | 2023 |
| One Embassy Motorworks Ltd | Wholesale & retail | 2019 |
| One Empire Security Ltd | Administrative & support | 2024 |
| One Halifax Road Ltd | Real estate | 2022 |
| One Love Manchester Project CIC | Information & communication | 2026 |
| One Love Takeaway Limited | Hospitality | 2019 |
| One SIX Eight Hospitality Ltd | Hospitality | 2024 |
| One Step Ahead Food Safety Ltd | Hospitality | 2018 |
| One Stop Claims Shop (UK) Ltd | Other services | 2014 |
| One Stop Joiners Ltd | Construction | 2026 |
| One Stop Mobile Tyres Ltd | Wholesale & retail | 2021 |
| One Technik Limited | Information & communication | 2013 |
| One Time Logistics Ltd | Transport & logistics | 2026 |
| One Union Solutions Ltd | Transport & logistics | 2021 |
| One Vision MTB Ltd | Arts & recreation | 2020 |
| Oneany Limited | Other services | 2017 |
| Onecore(Mcr). Ltd | Wholesale & retail | 2025 |
| Onehelix Limited | Energy | 2021 |
| Onestop Mega Ltd | Manufacturing | 2025 |
| Onestopgo Limited | Wholesale & retail | 2025 |
| Onetwo Didsbury Ltd | Arts & recreation | 2022 |
| Onevolt Ltd | Construction | 2021 |
| Oniks Events Limited | Arts & recreation | 2017 |
| Online 4 Baby Limited | Wholesale & retail | 2005 |
| Online Bathrooms Limited | Wholesale & retail | 2010 |
| Online FAN Page Ltd | Arts & recreation | 2025 |
| Online Fitness Clubs Ltd | Health & social care | 2023 |
| Online Tyres Limited | Wholesale & retail | 2017 |
| Onlinebridge Ltd | Wholesale & retail | 2025 |
| Only Decking Limited | Wholesale & retail | 2023 |
| Only Gill Ltd | Transport & logistics | 2020 |
| Onpoint Transport Limited | Transport & logistics | 2025 |
| Onside Secure Solutions Limited | Administrative & support | 2015 |
| Onthedot Minibus Hire Ltd | Transport & logistics | 2025 |
| Onus Facilities Management Ltd | Administrative & support | 2020 |
| Onyi Best Services Limited | Health & social care | 2022 |
| Onyx Comercio Ltd | Wholesale & retail | 2025 |
| Onyx Noir Studios Ltd | Other services | 2026 |
| Oomph Limited | Wholesale & retail | 2025 |
| Opal Dome Ltd | Wholesale & retail | 2018 |
| Opal Energy Limited | Construction | 2021 |
| Opal Transport Ltd | Transport & logistics | 2024 |