| 1 | O'Brien-Sill | | Oil & Gas Operator | Sep 2025 |
| 2 | O'Donnell Oil LLC | | Oil & Gas Operator | Aug 2025 |
| 3 | Oakland Petroleum Oper Co., Inc. | Kilgore, TX 75663 | Oil & Gas Operator | Nov 2025 |
| 4 | Occidental Of Elk Hills Inc. | | Oil & Gas Operator | Dec 2014 |
| 5 | Occidental Petroleum Corporation | | Oil & Gas Operator | Dec 2021 |
| 6 | Off Broadway Mineral Rights LLC | | Oil & Gas Operator | Oct 2024 |
| 7 | Ogle Petroleum Inc. | | Oil & Gas Operator | Dec 1989 |
| 8 | Oil Operators Inc. | | Oil & Gas Operator | Nov 2024 |
| 9 | Oil Production Management Inc | Squaw Valley, CA 93675-9725 | Oil & Gas Operator | Dec 2002 |
| 10 | Oil Securities Inc. | | Gas Operator | Dec 2003 |
| 11 | Oil Well Management Co. | | Oil & Gas Operator | Dec 1989 |
| 12 | Ojai Oil Company | Camarillo, CA 93012 | Oil & Gas Operator | Aug 2025 |
| 13 | Old Field Associates | | Oil & Gas Operator | Sep 2025 |
| 14 | Oleksandr Antonyuk | | Oil & Gas Operator | Dec 2018 |
| 15 | Omimex Petroleum Inc | Fort Worth, TX 76120 | Oil & Gas Operator | Aug 2022 |
| 16 | Optima Conservation Resources Exploration LLC | | Oil & Gas Operator | Sep 2025 |
| 17 | Oro Negro Inc. | | Oil & Gas Operator | Dec 2004 |
| 18 | Oryx Energy Company | Dallas, TX 75221 | Oil & Gas Operator | May 2002 |
| 19 | Output Exploration LLC | | Oil & Gas Operator | Dec 2006 |
| 20 | Oxy Petroleum Inc | | Oil & Gas Operator | Jan 1986 |
| 21 | Oxy USA Inc | Houston, TX 77046 | Oil & Gas Operator | Nov 2025 |