| 1 | O'Brien Energy Company | Shreveport, LA 71136 | Oil & Gas Operator | Sep 2019 |
| 2 | O. B. And Keith Whatley | | Oil Operator | Jul 1993 |
| 3 | O. B. Beck | | Oil Operator | Feb 1991 |
| 4 | O. B. Mobley | | Oil & Gas Operator | Jul 1989 |
| 5 | O. B. Whatley | | Oil Operator | Jul 1993 |
| 6 | O. F. H., Inc. | | Oil Operator | May 1990 |
| 7 | O.B. Whatley Oil Company, Inc. | | Oil Operator | Feb 2013 |
| 8 | O.B., Keith & Owen B. Whatley | | Oil Operator | Jul 1989 |
| 9 | Oak Valley Operating, LLC | The Woodlands, TX 77380 | Oil & Gas Operator | Jun 2015 |
| 10 | Ocean Energy, Inc. | Houston, TX 77002 | Oil & Gas Operator | Aug 2003 |
| 11 | Oil & Gas Producers, Inc. | Topeka, KS 66604 | Oil Operator | Jun 2004 |
| 12 | Oil Country Management, Inc. | | Oil & Gas Operator | Jan 2005 |
| 13 | Oil Lift, Incorporated | Metairie, LA 70002 | Oil & Gas Operator | Apr 2020 |
| 14 | Oil Reserves Corp Of America | | Oil & Gas Operator | May 2002 |
| 15 | Oil,Gas & Other Minerals, Inc. | | Oil & Gas Operator | Apr 1996 |
| 16 | Ok Oil Company, Inc. | Bivins, TX 75555 | Oil & Gas Operator | Feb 2013 |
| 17 | Ok-La-Tex Production, Inc. | | Oil & Gas Operator | Jan 1982 |
| 18 | Okie Crude Company | Tulsa, OK 74103 | Oil & Gas Operator | Jun 2025 |
| 19 | Omega Energy Incorporated | | Oil & Gas Operator | May 1998 |
| 20 | Ora Oil Company | | Oil & Gas Operator | May 2017 |
| 21 | Orx Resources, L.L.C. | Houston, TX 77067 | Oil & Gas Operator | Oct 2025 |
| 22 | Oryx Energy Company | Dallas, TX 75221 | Oil & Gas Operator | May 2002 |
| 23 | Owen Production Co | | Oil Operator | May 2001 |
| 24 | Oxy USA Inc | Houston, TX 77046 | Oil & Gas Operator | Oct 2025 |
| 25 | Ozark Production Company, Inc. | | Oil & Gas Operator | Jan 2025 |