| One-Dyas Exploration UK Limited | London | 2009 |
| One-Dyas Mariner Limited | Aberdeen | 2013 |
| One-Dyas North Sea Limited | Aberdeen | 2005 |
| One-Dyas Petroleum Limited | London | 2014 |
| One-Dyas UK Limited | London | 1998 |
| One-Foxx ENG Ltd | London | 2025 |
| Onegas West Limited | London | 2024 |
| Onesubsea Offshore Engineering Limited | London | 1987 |
| ONG Services Ltd | Peterhead | 2025 |
| Onnia Limited | London | 2002 |
| Onshore Oilfield Services Limited | Newark | 1996 |
| Onshore Petroleum Limited | Newark | 2017 |
| Onshore Production Services Limited | Newark | 2001 |
| Onsite Mechanical Engineers Limited | Dawlish | 2016 |
| Onsite Testing Services Ltd | Inverurie | 2012 |
| Onstream Limited | Gerrards Cross | 1978 |
| Onyx Global Ventures Ltd | Manchester | 2026 |
| Onyx Leisure Limited | Ilford | 2019 |
| Onyx Luxury Stone Ltd | Newport | 2023 |
| Opecon Limited | Worksop | 1989 |
| Operatio Limited | Inverurie | 2023 |
| Operations Integrity Ltd. | Stirling | 2012 |
| Operis Ltd | Great Yarmouth | 2025 |
| Ophir Energy Indonesia Limited | Henley-On-Thames | 2007 |
| Ophir Energy Limited | Henley-On-Thames | 2004 |
| Ophir Holdings & Services (UK) Limited | Henley-On-Thames | 2016 |
| Ophir Mexico Limited | Henley-On-Thames | 2016 |
| Ophrys International Limited | Colchester | 2018 |
| Opinari Limited | Epping | 2004 |
| Opsafe Ltd | Rayleigh | 2021 |
| Optima Measurement and Analytics Ltd | Wakefield | 2018 |
| Optima Minerals Holdings Ltd | London | 2026 |
| Optima Process Solutions Limited | Aberdeen | 2023 |
| Optima Solutions UK Limited | Aberdeen | 1999 |
| Optimal Angle Ltd | London | 2025 |
| Optimal Outsourcing Ltd | Wakefield | 2008 |
| Optimax Drilling Fluid Solutions UK Limited | London | 2016 |
| Optimum Access Services Limited | Berwick-Upon-Tweed | 2017 |
| Optimum Installations Limited | Bradford | 2002 |
| Opus Inspection Services Ltd | Great Yarmouth | 2018 |
| Opus OIL and GAS Limited | Guildford | 2016 |
| Oracle Gold Resources Ltd | Cambridge | 2020 |
| Oracle Power PLC | Cambridge | 2006 |
| Orapa Diamonds Public Limited Company | London | 2007 |
| Orbit Infinity Group Ltd | Northfleet | 2021 |
| Orcadian Energy (CNS) Ltd | London | 2014 |
| Orcadian Energy (SNS) Ltd | London | 2025 |
| Orcadian Energy PLC | London | 2021 |
| Orchard Lodge Investments Limited | Llantwit Major | 2020 |
| Orchid Interpretation Ltd | Chertsey | 2021 |
| Orcina Limited | Cumbria | 1986 |
| ORE & Metal International Limited | Winchester | 2022 |
| ORE Services Limited | London | 2024 |
| Oreilly Access Solutions Ltd | Glasgow | 2019 |
| Orica Blast & Quarry Surveys Limited | Lisburn | 2007 |
| Oriel Resources Limited | London | 2003 |
| Origin Composites Limited | Inverurie | 1991 |
| Origin Exploration Ltd | London | 2020 |
| Origin Mineral Group Ltd | Mitcham | 2026 |
| Origin Mining Limited | Truro | 2025 |
| Originstone & Critical Minerals Ltd | Chelmsford | 2026 |
| Oriole Bibemi Limited | Fareham | 2024 |
| Oriole Resources PLC | Fareham | 2005 |
| Orisoun Minerals & Resources Limited | Croydon | 2018 |
| Orizon Copi Global Mining Ltd | London | 2025 |
| Orkney Aggregates Limited | Kirkwall | 1999 |
| Orme Mineral Services Ltd | Norwich | 2016 |
| Ornan Services Ltd | Coulsdon | 2012 |
| ORO OIL and GAS Ltd | London | 2025 |
| Orosiba Holdings Ltd | London | 2026 |
| Orsett Quarry Limited | Witham | 1994 |
| Ortac Resources (UK) Limited | London | 2007 |
| Orton Services Limited | Morecambe | 2024 |
| Oryx Management (UK) Limited | London | 2012 |
| OS Access Ltd | Peterhead | 2019 |
| Osborne Energy Services Limited | Great Yarmouth | 2017 |
| Osborne Kruger Ltd | Surbiton | 2011 |
| OSC Services EA Limited | Lowestoft | 2012 |
| Oscar Access and Construction Limited | Washington | 2014 |
| Oscl Limited | Edinburgh | 2021 |
| OSE UK OIL Services & Energy Ltd | Enfield | 2024 |
| OSM Offshore Aberdeen Limited | Westhill | 2014 |
| OSS Holdings Limited | Luton | 2012 |
| Otherside Limited | Surrey | 2002 |
| OTR Holdings Limited | London | 1988 |
| Outside Inhouse Legal Limited | Aberdeen | 2017 |
| Overhall Aggregates Limited | Scunthorpe | 2018 |
| Overhall MIX Limited | Scunthorpe | 2023 |
| Overseas Turbo Repair Limited | London | 2019 |
| Owen (NDT) Inspection Services Limited | Blackpool | 2014 |
| Oxford Energy Consulting Ltd | Near Abingdon | 1997 |
| Oxleaze Ltd | Shilton, Burford | 2015 |
| Oxted Greystone Company Limited | London | 2007 |
| Oystergem Limited | Sandy | 2025 |
| O’Brien Aggregate Holding Limited | East Boldon | 2024 |
| O’Brien Aggregate Tuthill Limited | East Boldon | 2024 |
| P & D Designs (Inverclyde) Limited | Wemyss Bay | 2016 |
| P & H Baker Limited | Wolverhampton | 2017 |
| P & J Walls Limited | Antrim | 2015 |
| P & T Machinery Ltd | Armagh | 2003 |