| Odfjell Drilling Shetland Ltd | Aberdeen | 2009 |
| Odfjell RIG V Ltd | Eastleigh | 2018 |
| ODG Services Ltd | Dalkeith | 2025 |
| Odin Energy Ltd | London | 2013 |
| OEG Offshore Group Limited | Great Yarmouth | 2008 |
| OEG Offshore Limited | Kintore | 1985 |
| OES Oilfield Services (UK) Limited | Great Yarmouth | 2006 |
| Offerton Sand & Gravel Limited | Offerton Stockport | 1986 |
| Officepal Ltd | Windsor | 2011 |
| Offington Resources | Iver | 2019 |
| Offshore Biker Limited | Inverness | 2024 |
| Offshore Hydrographic Processing Ltd | Harleston | 2004 |
| Offshore Installation Contractors Ltd | Montrose | 2016 |
| Offshore Marine Contractors Limited | Great Yarmouth | 1991 |
| Offshore Marine Limited | Maidstone | 2015 |
| Offshore Positioning Solutions Limited | Taunton | 2009 |
| Offshore Products International Limited | Ellon | 1999 |
| Offshore Resource Management Ltd | Isle Of Lewis | 2025 |
| Offshore Services Ltd | Paisley | 2020 |
| Offshore Support Vessels IV Limited | Teesside | 1966 |
| Offshore Support Vessels IX Limited | Aberdeen | 1999 |
| Offshore Support Vessels V Limited | Teesside | 2001 |
| Offshore Support Vessels VI Limited | Aberdeen | 2007 |
| Offshore Support Vessels VII Limited | Aberdeen | 2007 |
| Offshore Survey Consult Limited | Edinburgh | 2023 |
| Offshore Terminals & Tankers Ltd. | | 1975 |
| Offsol Ltd | Peterhead | 2018 |
| Ogbanufe Offshore Services Limited | Great Yarmouth | 2014 |
| Ogbonna Onyewuchi Investment Limited | London | 2017 |
| Ogilvie Geoscience Limited | Aberdeen | 2016 |
| Ognr Compliance Limited | Middlesbrough | 2018 |
| Ognyanova Ltd | Glannau Dyfrdwy | 2025 |
| OH Quarries Ltd | Belfast | 2024 |
| OIL & GAS (Staff) Ltd | Whitley Bay | 2022 |
| OIL & GAS Academy (UK) Limited | Southsea | 2007 |
| OIL & GAS Advisors Limited | Lingfield | 2010 |
| OIL & GAS Broker International Ltd | London | 2026 |
| OIL & GAS Hsse Services Limited | Betchworth | 2007 |
| OIL & GAS Ltd | Durham | 2019 |
| OIL & GAS Process Services Ltd | Aberdeen | 2024 |
| OIL & GAS Technical Services Ltd | Edinburgh | 2010 |
| OIL and GAS Engineering Ltd | Glasgow | 2013 |
| OIL and GAS Outlook Consulting Group Ogocg Limited | London | 2017 |
| OIL and GAS Plus Limited | Manchester | 2023 |
| OIL and GAS Resourcing Ltd | Fareham | 2012 |
| OIL and GAS Services (Richmond) Limited | Richmond | 2025 |
| OIL Black Offshore Limited | Inverurie | 1997 |
| OIL Maintenance Systems Ltd | Leeds | 2017 |
| OIL Patch Logistics Ltd | Richmond | 2017 |
| OIL States Industries (UK) Limited | Aberdeen | 1996 |
| Oilbiz UK Limited | London | 2020 |
| Oildynamix Ltd | Aberdeen | 2013 |
| Oilexe Limited | Grimsby | 2010 |
| Oilfield Consultants International Ltd | Hayling Island | 1990 |
| Oilfield Containers Limited | Great Yarmouth | 2007 |
| Oilfield Energy Services Ltd | Great Yarmouth | 2017 |
| Oilfield Logistics Management Limited | Norwich | 2001 |
| Oilfield Strategic Solutions Ltd | Mansfield | 2013 |
| Oilfield Tool Leasing | London | 2007 |
| Oilserv (UK) Ltd. | Yarm | 2009 |
| Oiltech Group for OIL Services & Construction Ltd | Wembley | 2020 |
| Oiltex Ltd | Montrose | 2005 |
| OJC Contracts Limited | Armagh | 2014 |
| OJH Geological Limited | Bedford | 2017 |
| OK Energy Limited | Oxford | 2010 |
| Okans Global Energy Limited | Clacton-On-Sea | 2019 |
| Okasitu-Rk Ltd | Croydon | 2023 |
| Oklonda International Chemicals UK Co., Ltd | West Midlands, England | 2026 |
| Okvin Group Technology Ltd | Hinckley | 2021 |
| OL Power Ltd | Market Harborough | 2022 |
| OLA Geoscience Ltd | Belfast | 2017 |
| Olamac Limited | Stonehaven | 2012 |
| Olax Engineering Consulting Ltd | London | 2020 |
| Old Cottage Field Limited | Lichfield | 2021 |
| Old Place Holdings Ltd | Stockport | 2015 |
| Oldco (04848025) Limited | London | 2003 |
| Oleum Analytics Limited | Aberdeen | 2023 |
| Oleum Holdings Ltd | Chigwell | 2025 |
| Olexon Ltd | Hitchin | 2025 |
| Oligo Limited | Beaconsfield | 2012 |
| Olion International B.V Ltd | Cardiff | 2023 |
| Ollar Engineering Limited | Callander | 2014 |
| Olly's Materials Services Ltd | Aberdeen | 2018 |
| Olvia RMM International Ltd | London | 2019 |
| OM Mining Ltd | Birmingham | 2024 |
| Omagh Minerals Limited | Belfast | 1990 |
| Omagh Sand and Gravel Ltd | Omagh | 2025 |
| Omega Geoservices and Consultancy Limited | Aberdeen | 2014 |
| Omega Well Intervention Limited | Aberdeen | 1998 |
| Omega-Butler International Bio-Fuels Ltd | Kent | 2018 |
| Omenatic Limited | Caterham | 2023 |
| Omex Mining Ltd | London | 2023 |
| Omnex Energy Ltd | London | 2026 |
| Omnex Group Ltd | London | 2026 |
| Omnex Mining Ltd | London | 2026 |
| Omnienergix Limited | London | 2018 |
| OMR Sands Limited | Crewe | 1995 |
| One Global Energy UK Limited | London | 2022 |
| One-Dyas E&P Limited | London | 2000 |
| One-Dyas EOG Limited | Aberdeen | 1902 |