| 401 | Olympic Production Company | | Oil & Gas Operator | Jul 1994 |
| 402 | Olympic Resources Co | Wooster, OH 44691 | Oil & Gas Operator | Dec 2000 |
| 403 | Olympus Energy, LLC | Canonsburg, PA 15317-7536 | Gas Operator | Jun 2025 |
| 404 | Olympus Prop Inc | Franklin Park, IL 60131-1418 | Oil & Gas Operator | Dec 1997 |
| 405 | Omac Oil & Gas Co | Newark, OH 43055 | Oil & Gas Operator | Dec 1990 |
| 406 | Omar Operating Company | Midland, TX 79702 | Oil & Gas Operator | Sep 2013 |
| 407 | Omber Petroleum & Energy Corp. | New Braunfels, TX 78131 | Oil & Gas Operator | Dec 2025 |
| 408 | Omeara Bros. | | Oil & Gas Operator | Oct 1982 |
| 409 | Omega Alpha Resources, LLC | Dallas, TX 75219 | Oil Operator | Dec 2025 |
| 410 | Omega Energy Corp. | Graham, TX 76450 | Oil & Gas Operator | Dec 2025 |
| 411 | Omega Energy Incorporated | | Oil & Gas Operator | May 1998 |
| 412 | Omega Energy, LLC | Corpus Chrsti, TX 78418 | Oil & Gas Operator | Dec 2025 |
| 413 | Omega Enterprise | | Oil Operator | Sep 1990 |
| 414 | Omega Oil Corp | Lafayette, IN 47903 | Oil & Gas Operator | Dec 1987 |
| 415 | Omega Operating, Inc. | Odessa, TX 79769 | Oil & Gas Operator | Mar 2001 |
| 416 | Omega Resources Inc | | Oil & Gas Operator | Nov 1998 |
| 417 | Omimex Canada, LTD. | Fort Worth, TX 76120-3608 | Oil & Gas Operator | Sep 2025 |
| 418 | Omimex Energy, Inc. | Fort Worth, TX 76120 | Oil & Gas Operator | Aug 2025 |
| 419 | Omimex International Corp | Fort Worth, TX 76103-2300 | Oil & Gas Operator | Aug 2025 |
| 420 | Omimex Petroleum Inc | Fort Worth, TX 76120 | Oil & Gas Operator | Aug 2022 |
| 421 | Omimex Petroleum Inc (Co) | Newcastle, WY 82701 | Oil & Gas Operator | May 2001 |
| 422 | Omimex Petroleum Inc (De) | Fort Worth, TX 76120 | Oil & Gas Operator | Aug 2020 |
| 423 | Omni E & P, Inc. | San Antonio, TX 78229 | Oil Operator | Apr 2010 |
| 424 | Omni Oil & Gas, Inc. | San Antonio, TX 78229 | Oil & Gas Operator | Oct 2017 |
| 425 | Omni Operating Company | Osage, WY 82723 | Oil & Gas Operator | Sep 2001 |
| 426 | Omni Operating Company(A) | Dallas, TX 75370 | Oil & Gas Operator | Dec 2025 |
| 427 | Omni Petroleum Corporation | Austin, TX 78738 | Oil & Gas Operator | Dec 2025 |
| 428 | Omr Drilling & Acquisition LLC | Albany, KY 42602 | Oil Operator | Dec 2015 |
| 429 | Omta Operating LLC | Corpus Chrsti, TX 78418 | Oil & Gas Operator | Dec 2025 |
| 430 | On Pointe Operating LLC | Jones, OK 73049 | Oil & Gas Operator | Dec 2025 |
| 431 | On-Line Oil Inc | Oklahoma City, OK 73105-5208 | Oil & Gas Operator | Mar 2015 |
| 432 | Onapa Gas Company LLC | Tulsa, OK 74136-7253 | Gas Operator | Nov 2019 |
| 433 | Onaroll Operating LLC | Levelland, TX 79336 | Oil Operator | Dec 2025 |
| 434 | Onco Petroleum USA Inc | | Oil Operator | Dec 2009 |
| 435 | One Epoch Energy, Inc. | Abilene, TX 79604 | Oil & Gas Operator | Mar 2008 |
| 436 | One Nickel Operating LLC | Ashland, MA 01721 | Oil Operator | Dec 2025 |
| 437 | One Source Financial Services Inc. | McAllen, TX 78501 | Gas Operator | Dec 2019 |
| 438 | One Tec Operating, LLC | Houston, TX 77002 | Oil & Gas Operator | Nov 2007 |
| 439 | One World Operating, LLC | Tampa, FL 33602 | Oil & Gas Operator | Dec 2025 |
| 440 | Oneenergy Partners Operating, LLC | Houston, TX 77019 | Oil & Gas Operator | Nov 2018 |
| 441 | Oneida Energy Corporation | Tulsa, OK 74137 | Oil & Gas Operator | Jul 2003 |
| 442 | Oneok Bushton Proces | | Gas Operator | Mar 2007 |
| 443 | Oneok Energy Service | | Gas Operator | Dec 2008 |
| 444 | Oneok Exploratio | | Oil & Gas Operator | Mar 1995 |
| 445 | Oneok Exploration Company | Tulsa, OK 74102 | Oil & Gas Operator | Feb 2017 |
| 446 | Oneok Field Services Company | Tulsa, OK 74103 | Oil & Gas Operator | Jul 2022 |
| 447 | Oneok Gas Processing LLC | Tulsa, OK 74103 | Oil & Gas Operator | May 2018 |
| 448 | Oneok Gas Transportation Co | Tulsa, OK 74121-2089 | Oil & Gas Operator | Oct 2014 |
| 449 | Oneok Resources Company | Tulsa, OK 74102 | Oil & Gas Operator | Feb 2017 |
| 450 | Onexxx Prod & Exploration Corp | Jeannette, PA 15644-9691 | Gas Operator | Dec 2024 |
| 451 | Onnen, Gerhard E. | Yates Center, KS 66783 | Oil Operator | Feb 2002 |
| 452 | Onpoint Oil & Gas LLC | Oklahoma City, OK 73170-4848 | Oil & Gas Operator | Aug 2025 |
| 453 | Onshore Exp Cor-La Nat Res Inc | | Oil & Gas Operator | Oct 1978 |
| 454 | Onshore Pipeline Const Co Inc | | Oil & Gas Operator | Aug 1998 |
| 455 | Onshore Royalties LLC | Houston, TX 77269 | Oil & Gas Operator | Sep 2025 |
| 456 | Onshore, L.L.C. | Wichita, KS 67205 | Oil & Gas Operator | Dec 2009 |
| 457 | Onyx Energy Corporation | Oklahoma City, OK 73157 | Oil & Gas Operator | May 1994 |
| 458 | Onyx Energy Dev Of Texas Inc. | | Oil Operator | Dec 1996 |
| 459 | Onyx Energy Of Texas Inc. | | Oil Operator | Dec 1996 |
| 460 | Onyx Industries LTD, Inc. | Wichita Falls, TX 76308 | Oil & Gas Operator | Aug 2003 |
| 461 | Onyx Int'L Exploration LLC | | Oil & Gas Operator | Dec 2015 |
| 462 | Onyx Producing Company, Inc. | Pampa, TX 79066 | Oil & Gas Operator | Mar 2008 |
| 463 | Onyx Properties Corp | Oklahoma City, OK 73114 | Oil & Gas Operator | May 2006 |
| 464 | Onyxx Energy LLC | | Oil & Gas Operator | Mar 2017 |
| 465 | Onyxx Oil Corpor | | Oil Operator | Jul 1992 |
| 466 | Oolite Energy Corp | Amarillo, TX 79105 | Oil & Gas Operator | Apr 2022 |
| 467 | Oonk, Harvey D. | Clymer, NY 14724 | Gas Operator | Dec 2024 |
| 468 | Opal Energy, Inc. | Katy, TX 77494 | Oil & Gas Operator | Jun 2009 |
| 469 | Opal Oil Inc. | | Oil Operator | Sep 2021 |
| 470 | Opal Operating Company LLC | Edmond, OK 73013 | Oil & Gas Operator | Dec 2025 |
| 471 | Opal Resources Operating Co.,LLC | Cypress, TX 77429 | Oil & Gas Operator | Jan 2019 |
| 472 | Opco LLC | Chase, KS 67524 | Oil Operator | Dec 2019 |
| 473 | Opco Oil & Gas Co. | Dallas, TX 75231 | Oil & Gas Operator | Aug 2003 |
| 474 | Opdycke Lease Partners, A General Partnership | Russell, KS 67665 | Oil Operator | Jun 2025 |
| 475 | Opef Partnership, A General Partnership | Overland Park, KS 66221 | Oil Operator | Dec 2011 |
| 476 | Open Door Holdings Inc | Surprise, AZ 85387-2015 | Oil & Gas Operator | Feb 2024 |
| 477 | Open Door Productions Inc | Muskogee, OK 74401-1032 | Oil & Gas Operator | Apr 2020 |
| 478 | Open Flow Gas Supply Corp | Du Bois, PA 15801-2442 | Oil & Gas Operator | Dec 2024 |
| 479 | Open Range Energy LLC | Jones, OK 73049-8744 | Oil & Gas Operator | Mar 2025 |
| 480 | Operating Engineers Local 17 Training Fund | Lake VIew, NY 14085 | Gas Operator | Dec 2024 |
| 481 | Operator Unknown | Charleston, WV 25304-0000 | Oil & Gas Operator | Dec 2021 |
| 482 | Opes Production Inc | Buffalo, WY 82834 | Gas Operator | Mar 2015 |
| 483 | Opex Energy, LLC | Houston, TX 77042 | Oil & Gas Operator | Mar 2008 |
| 484 | Opmi Operating Company | Houston, TX 77057 | Oil & Gas Operator | Jun 2008 |
| 485 | Ops Group Limited | Houston, TX 77060 | Oil & Gas Operator | Jun 2014 |
| 486 | Optima Conservation Resources Exploration LLC | | Oil & Gas Operator | Oct 2025 |
| 487 | Optima Exploration LLC | Tulsa, OK 74116-5664 | Oil & Gas Operator | Mar 2015 |
| 488 | Optimistic Oil Co. | | Oil & Gas Operator | Dec 1991 |
| 489 | Optimum Exploration LLC | Dallas, TX 75254 | Oil & Gas Operator | Dec 2025 |
| 490 | Opubco Resources Of TX Inc | Dallas, TX 75231 | Oil & Gas Operator | Sep 2005 |
| 491 | Ora Oil Company | | Oil & Gas Operator | May 2017 |
| 492 | Oracle Oil, LLC | | Oil & Gas Operator | Jul 2013 |
| 493 | Oracle Resources Oklahoma LP | Dallas, TX 75240-5751 | Oil & Gas Operator | Feb 2008 |
| 494 | Orange Petroleum Co. | Orangefield, TX 77639 | Oil & Gas Operator | May 1995 |
| 495 | Orangefield East Corp | Dallas, TX 75382 | Oil Operator | Dec 2025 |
| 496 | Orangefield Operating Corporation | Dallas, TX 75382 | Oil Operator | Dec 2025 |
| 497 | Orason Resources Inc | Tulsa, OK 74169-1225 | Oil & Gas Operator | Nov 2006 |
| 498 | Orb Exploration, L.L.C. | | Oil & Gas Operator | Jun 2017 |
| 499 | Orbis Energy, L.L.C. | Austin, TX 78746 | Oil & Gas Operator | Oct 2001 |
| 500 | Orbit Crude, Inc. | Midland, TX 79710 | Oil & Gas Operator | Dec 2025 |