| 1 | O G & E | Oklahoma City, OK 73124 | Oil & Gas Operator | Feb 2014 |
| 2 | O'Brien Resources LLC | Shreveport, LA 71136 | Oil Operator | Nov 2017 |
| 3 | Oak Resources Inc | Tulsa, OK 74153 | Oil & Gas Operator | Aug 2013 |
| 4 | Oexco, Inc. | Tulsa, OK 74136 | Oil & Gas Operator | Mar 2015 |
| 5 | Ofs-Tulsa Corporation | Fort Worth, TX 76102 | Oil & Gas Operator | Feb 1997 |
| 6 | Oklahoma Oil & Gas MGMT Inc | Edmond, OK 73083-6786 | Oil & Gas Operator | Oct 2024 |
| 7 | Oklahoma Oil Tactics Inc | Edmond, OK 73003-1351 | Oil & Gas Operator | Mar 2015 |
| 8 | Oklahoma Tank Service Inc | Oklahoma City, OK 73143 | Oil Operator | Aug 1997 |
| 9 | Okland Oil Company | Oklahoma City, OK 73102-9022 | Oil & Gas Operator | Oct 2024 |
| 10 | Okt Petroleum Company Inc | Oklahoma City, OK 73118-7425 | Oil & Gas Operator | Oct 2013 |
| 11 | Oneok Exploration Company | Tulsa, OK 74102 | Oil & Gas Operator | Feb 2017 |
| 12 | Oneok Resources Company | Tulsa, OK 74102 | Oil & Gas Operator | Feb 2017 |
| 13 | Onyx Energy Corporation | Oklahoma City, OK 73157 | Oil & Gas Operator | May 1994 |
| 14 | Onyx Properties Corp | Oklahoma City, OK 73114 | Oil & Gas Operator | May 2006 |
| 15 | Orca Operating Company LLC | Houston, TX 77056 | Oil & Gas Operator | Oct 2024 |
| 16 | Oryx Crude Trading & Transporation LP | Dallas, TX 75221 | Oil & Gas Operator | Jun 1999 |
| 17 | Outback Petroleum Inc | Moore, OK 73160-9998 | Oil & Gas Operator | Mar 2015 |
| 18 | Ovation Energy II, LP | Houston, TX 77279 | Oil & Gas Operator | Jun 2011 |
| 19 | Oxford Exploration Company | Denver, CO 80237 | Oil & Gas Operator | Aug 1995 |
| 20 | Oxy USA Inc | Houston, TX 77046 | Oil & Gas Operator | Dec 2024 |