| 1 | O G & E | Oklahoma City, OK 73124 | Oil & Gas Operator | Feb 2014 |
| 2 | Oakland Petroleum Oper Co., Inc. | Kilgore, TX 75663 | Oil & Gas Operator | Feb 2026 |
| 3 | Occidental Pet Corp | Houston, TX 77046 | Oil & Gas Operator | Oct 1997 |
| 4 | Oklahoma Oil Tactics Inc | Edmond, OK 73003-1351 | Oil & Gas Operator | Mar 2015 |
| 5 | Okland Oil Company | Oklahoma City, OK 73102-9022 | Oil & Gas Operator | Dec 2025 |
| 6 | Old Dominion Oil Corporation | Oklahoma City, OK 73107 | Oil & Gas Operator | Apr 2009 |
| 7 | Oneok Field Services Company | Tulsa, OK 74103 | Oil & Gas Operator | Jul 2022 |
| 8 | Oneok Gas Processing LLC | Tulsa, OK 74103 | Oil & Gas Operator | May 2018 |
| 9 | Oneok Resources Company | Tulsa, OK 74102 | Oil & Gas Operator | Feb 2017 |
| 10 | Opal Operating Company LLC | Edmond, OK 73013 | Oil & Gas Operator | Feb 2026 |
| 11 | Overflow Energy, LLC | Perryton, TX 79070 | Oil & Gas Operator | Feb 2026 |
| 12 | Ovintiv Mid-Continent Inc. | The Woodlands, TX 77380-2764 | Oil & Gas Operator | Dec 2025 |
| 13 | Oxley Petroleum | Tulsa, OK 74119-3609 | Oil & Gas Operator | Jan 2020 |
| 14 | Oxley Petroleum Company | Oklahoma City, OK 73118-1044 | Oil & Gas Operator | Sep 2017 |
| 15 | Oxy USA Inc | Houston, TX 77046 | Oil & Gas Operator | Feb 2026 |