| OIL & GAS Technical Services Ltd | Mining & quarrying | 2010 |
| OIL Field Turbine Consultants UK Ltd. | Professional & scientific | 2011 |
| OIL Marine Technology LP | | 2016 |
| OIL Pollution Services Limited | Administrative & support | 2006 |
| OIL Recovery Investments Limited | Professional & scientific | 2013 |
| OIL Trade Spectrum LP | | 2015 |
| OIL Trading Europe LP | | 2018 |
| OIL Trans Rail L.P. | | 2008 |
| OIL, GAS & Power Co-Invest 2006-08 LP | | 2006 |
| Oila Ltd | Real estate | 2021 |
| Oilfield Chemical Additives Limited | Manufacturing | 1996 |
| Oilfield Chemical Technology Limited | Professional & scientific | 1992 |
| Oilfield Industries LP | | 2013 |
| Oilfield Machinery Limited | Wholesale & retail | 2010 |
| Oils and Feed Production LP | | 2019 |
| Oilseeds Trading LP | | 2013 |
| Oiltrade Marine LLP | | 2009 |
| OIR & Co Ltd | Hospitality | 2026 |
| Oishigami Limited | Manufacturing | 2024 |
| Oister Group LP | | 2016 |
| Ojfk Orthopaedics Limited | Health & social care | 2024 |
| OJG Property Ltd | Real estate | 2026 |
| OJM Investment Ltd | Real estate | 2025 |
| Ojpd Ltd | Construction | 2023 |
| OJU Ologe Ltd | Other services | 2025 |
| OK International Research LP | | 2016 |
| OK Positive Ltd | Information & communication | 2019 |
| Okavango Investments Limited | Administrative & support | 2016 |
| Okawango LP | | 2013 |
| Okay Limited | Real estate | 1993 |
| Okay Monday LP | | 2017 |
| Okkam LP | | 2014 |
| Okko Alliance LP | | 2011 |
| Okko Capital LP | | 2015 |
| Oklib Ltd | Professional & scientific | 2022 |
| Okni-Hws Limited | Wholesale & retail | 2024 |
| Okoa-Reb Limited | Other services | 2005 |
| Okola Care Ltd | Public administration | 2022 |
| Okoroafro Limited | Wholesale & retail | 2023 |
| Okpu and Family Farms Ltd | Agriculture & fishing | 2021 |
| Okta Resourcing Limited | Administrative & support | 2014 |
| Oktiq LP | | 2018 |
| Okuzo Limited | Wholesale & retail | 2020 |
| OLA Kala Foods Ltd | Wholesale & retail | 2019 |
| Olav Tryggvasson LP | | 2014 |
| Olbia Food LP | | 2015 |
| Old Blossom Limited | Hospitality | 2025 |
| Old Capital Ventures L.P. | | 2012 |
| Old City LP | | 2013 |
| Old College Capital GP Limited | Administrative & support | 2011 |
| Old College Capital Holdings Limited | Financial services | 1985 |
| Old College Capital LP | | 2011 |
| Old College Capital Strategic Investments LP | | 2012 |
| Old Dogs New Tricks Ltd | Administrative & support | 2025 |
| Old Fashioned Hospitality Ltd | Hospitality | 2023 |
| Old Green Stud Limited | Agriculture & fishing | 2017 |
| Old Mill on the Spey Ltd | Hospitality | 2015 |
| Old PAL Ltd | Hospitality | 2024 |
| Old Poison Ltd | Administrative & support | 2017 |
| Old Prints & New Ideas L.P. | | 2011 |
| Old Rugged Cross Limited | Manufacturing | 2026 |
| Old School Roofing & Building Ltd | Construction | 2025 |
| Old SL32307 LP | | 2017 |
| Old Stables Trading Ltd | Real estate | 2024 |
| Old Town Barber Ltd | Other services | 2023 |
| Old Town Blending Company Limited | Professional & scientific | 1983 |
| Old Town Brewing Company Ltd | Information & communication | 2012 |
| Old Town Express Ltd | Wholesale & retail | 2023 |
| Old Town Hideways Ltd | Real estate | 2026 |
| Old Town Holiday Lets Ltd | Real estate | 2019 |
| Old Town Threads Ltd | Wholesale & retail | 2025 |
| Oldbell Kitchen Limited | Hospitality | 2025 |
| Oldbrook International Corp. | | 2009 |
| Oldbrook Investments L.P. | | 2011 |
| Oldcrest Invest L.P. | | 2015 |
| Olde Edinburgh Ltd | Wholesale & retail | 2019 |
| Olde Edinburgh Scotland Limited | Wholesale & retail | 2026 |
| Olde SLP | | 1998 |
| Oldempire Associates L.P. | | 2010 |
| Oldenburg Trading LP | | 2013 |
| Oldest Pet Store 1953 Ltd | Wholesale & retail | 2014 |
| Oldhill Universal L.P. | | 2014 |
| Oldline Trading L.P. | | 2015 |
| Oldmall Trading LP | | 2014 |
| Oldstar Investments L.P. | | 2008 |
| Oldstone International L.P. | | 2010 |
| Oldstyle Investments L.P. | | 2011 |
| Oldway Enterprises L.P. | | 2008 |
| Oldway Management L.P. | | 2011 |
| Olea Campus Limited | Wholesale & retail | 2026 |
| Olemda Ltd | Real estate | 2018 |
| Olenad Management LP | | 2014 |
| Oleo Trading LP | | 2016 |
| Oleon Capital LP | | 2015 |
| Oleron Mill LP | | 2013 |
| Oletta LP | | 2013 |
| OLG Barbara Sturm, L.P. | | 2019 |
| Olifravan Limited | Hospitality | 2016 |
| Olima Ship Services LP | | 2017 |
| Olimp Invest LP | | 2015 |