| 1 | O G & E | Oklahoma City, OK 73124 | Oil & Gas Operator | Feb 2014 |
| 2 | Okie Crude Company | Tulsa, OK 74103 | Oil & Gas Operator | Sep 2024 |
| 3 | Oklahoma & Northwestern Co. | Oklahoma City, OK 73102 | Oil & Gas Operator | Jan 1997 |
| 4 | Oklahoma Equipment & Salvage | Oklahoma City, OK 73120-5903 | Oil & Gas Operator | Mar 2015 |
| 5 | Oklahoma Oil & Gas MGMT Inc | Edmond, OK 73083-6786 | Oil & Gas Operator | Oct 2024 |
| 6 | Okland Oil Company | Oklahoma City, OK 73102-9022 | Oil & Gas Operator | Oct 2024 |
| 7 | Old Dominion Oil Corporation | Oklahoma City, OK 73107 | Oil & Gas Operator | Apr 2009 |
| 8 | Oneok Exploration Company | Tulsa, OK 74102 | Oil & Gas Operator | Feb 2017 |
| 9 | Oneok Field Services Company | Tulsa, OK 74103 | Oil & Gas Operator | Jul 2022 |
| 10 | Oneok Gas Transportation Co | Tulsa, OK 74121-2089 | Oil & Gas Operator | Oct 2014 |
| 11 | Oneok Resources Company | Tulsa, OK 74102 | Oil & Gas Operator | Feb 2017 |
| 12 | Opal Operating Company LLC | Oklahoma City, OK 73142 | Oil & Gas Operator | Dec 2024 |
| 13 | Overflow Energy, LLC | Booker, TX 79005 | Oil & Gas Operator | Dec 2024 |
| 14 | Ovintiv Mid-Continent Inc. | The Woodlands, TX 77380-2764 | Oil & Gas Operator | Oct 2024 |
| 15 | Ovintiv USA Inc. | Denver, CO 80202 | Oil & Gas Operator | Dec 2024 |
| 16 | Oxley Petroleum | Tulsa, OK 74119-3609 | Oil & Gas Operator | Jan 2020 |
| 17 | Oxley Petroleum Company | Oklahoma City, OK 73118-1044 | Oil & Gas Operator | Sep 2017 |
| 18 | Oxy USA Inc | Houston, TX 77046 | Oil & Gas Operator | Dec 2024 |