| 1 | O'Blue Corp. | Kerrville, TX 78028 | Oil & Gas Operator | Mar 1998 |
| 2 | O'Neal Oil & Gas Co | Fort Worth, TX 76107-1479 | Oil & Gas Operator | Mar 2025 |
| 3 | O'Neal Oil Company | Nocona, TX 76255 | Oil & Gas Operator | Jun 2025 |
| 4 | O'Neal, Lindell | Nocona, TX 76255 | Oil & Gas Operator | Mar 2025 |
| 5 | Oak Resources Inc | Tulsa, OK 74153 | Oil & Gas Operator | Aug 2013 |
| 6 | Offield Lease Operations LLC | Wilson, OK 73463 | Oil & Gas Operator | May 2023 |
| 7 | Oilco Energy Inc | Duncan, OK 73533 | Oil & Gas Operator | Mar 2015 |
| 8 | Oilwell Operators, Inc. | Pampa, TX 79066 | Oil & Gas Operator | Jun 2025 |
| 9 | Oklahoma Liquid Resources Inc | Collinsville, OK 74021 | Oil & Gas Operator | Feb 2025 |
| 10 | Oklahoma Oil & Gas MGMT Inc | Edmond, OK 73083-6786 | Oil & Gas Operator | Feb 2025 |
| 11 | Oklahoma Prime Energy LLC | Ardmore, OK 73401 | Oil Operator | May 2020 |
| 12 | Oklahoma Tank Service Inc | Oklahoma City, OK 73143 | Oil Operator | Aug 1997 |
| 13 | Old Dominion Oil Corporation | Oklahoma City, OK 73107 | Oil & Gas Operator | Apr 2009 |
| 14 | Oleum Incorporated | | Oil & Gas Operator | Mar 1996 |
| 15 | Oneok Gas Transportation Co | Tulsa, OK 74121-2089 | Oil & Gas Operator | Oct 2014 |
| 16 | Oneok Resources Company | Tulsa, OK 74102 | Oil & Gas Operator | Feb 2017 |
| 17 | Oracle Resources Oklahoma LP | Dallas, TX 75240-5751 | Oil & Gas Operator | Feb 2008 |
| 18 | Outback Petroleum Inc | Moore, OK 73160-9998 | Oil & Gas Operator | Mar 2015 |
| 19 | Ovintiv Mid-Continent Inc. | The Woodlands, TX 77380-2764 | Oil & Gas Operator | Mar 2025 |
| 20 | Oxy USA Inc | Houston, TX 77046 | Oil & Gas Operator | Jun 2025 |