| 301 | Oklahoma Equipment & Salvage | Oklahoma City, OK 73120-5903 | Oil & Gas Operator | Mar 2015 |
| 302 | Oklahoma Gas & Oil Corp | | Oil Operator | Oct 2018 |
| 303 | Oklahoma Liquid Resources Inc | Collinsville, OK 74021 | Oil & Gas Operator | Feb 2026 |
| 304 | Oklahoma Marlin Oil Corporation | Oklahoma City, OK 73113 | Oil & Gas Operator | Feb 2020 |
| 305 | Oklahoma Natural Resources Inc | Woodward, OK 73801 | Oil & Gas Operator | Sep 1996 |
| 306 | Oklahoma O & G Royalties Inc | Duncan, OK 73533 | Oil & Gas Operator | Nov 2000 |
| 307 | Oklahoma Oil & Gas MGMT Inc | Edmond, OK 73083-6786 | Oil & Gas Operator | Mar 2026 |
| 308 | Oklahoma Oil And Gas Resources LLC | Guthrie, OK 73044-8095 | Oil & Gas Operator | Mar 2026 |
| 309 | Oklahoma Oil Company | Dallas, TX 75206 | Oil & Gas Operator | Aug 2013 |
| 310 | Oklahoma Oil Tactics Inc | Edmond, OK 73003-1351 | Oil & Gas Operator | Mar 2015 |
| 311 | Oklahoma Operating Company | Midland, TX 79708 | Oil & Gas Operator | Jun 2026 |
| 312 | Oklahoma Operating, LLC | Holliday, TX 76366 | Oil Operator | Nov 2003 |
| 313 | Oklahoma Petroleum MGMT Corp | Tulsa, OK 74119 | Oil & Gas Operator | Jun 1993 |
| 314 | Oklahoma Prime Energy LLC | Ardmore, OK 73401 | Oil Operator | May 2020 |
| 315 | Oklahoma RB Operating Company | Midland, TX 79701 | Oil & Gas Operator | Jan 1998 |
| 316 | Oklahoma Rig & Salvage Inc | Oklahoma City, OK 73114 | Oil & Gas Operator | Mar 2015 |
| 317 | Oklahoma SWD Systems Inc | Odessa, TX 79762-8132 | Oil & Gas Operator | Mar 2026 |
| 318 | Oklahoma Tank Service Inc | Oklahoma City, OK 73143 | Oil Operator | Aug 1997 |
| 319 | Okland Oil Company | Oklahoma City, OK 73102-9022 | Oil & Gas Operator | Apr 2026 |
| 320 | Oklaoleum Oil And Gas Inc | Bristow, OK 74010-0057 | Oil Operator | Jan 2016 |
| 321 | Oklatex Energy, Inc. | Beaumont, TX 77702 | Oil Operator | Jun 2026 |
| 322 | Okmar Oil Compan | | Oil & Gas Operator | Feb 1995 |
| 323 | Okmar Oil Company | Marietta, OH 45750 | Oil & Gas Operator | Jun 2005 |
| 324 | Okreek Oil Company | Weatherford, TX 76086 | Oil & Gas Operator | Jun 2026 |
| 325 | Okt Petroleum Company Inc | Oklahoma City, OK 73118-7425 | Oil & Gas Operator | Oct 2013 |
| 326 | Okt Resources, LLC | Edmond, OK 73013 | Oil & Gas Operator | Jun 2026 |
| 327 | Oktex Operating LLC | Edmond, OK 73013-1961 | Oil & Gas Operator | May 2020 |
| 328 | Oky Investments, Inc. | Edmond, OK 73013 | Oil & Gas Operator | Sep 2021 |
| 329 | Ol' Boys Operations LLC | Pampa, TX 79065 | Oil & Gas Operator | Jun 2026 |
| 330 | Ol' Oily LLC | | Oil Operator | Feb 2026 |
| 331 | Old Acres Farm, Inc. | Perry, NY 14530 | Gas Operator | Dec 2024 |
| 332 | Old Cedron, L.L.C. | Lubbock, TX 79410 | Oil & Gas Operator | May 2004 |
| 333 | Old Chief Oil, LLC | San Antonio, TX 78205 | Oil Operator | Jun 2026 |
| 334 | Old Dominion Oil Corporation | Oklahoma City, OK 73107 | Oil & Gas Operator | Apr 2009 |
| 335 | Old Field Associates | | Oil & Gas Operator | Mar 2026 |
| 336 | Old MTN Gas Co Inc | Pleasantville, PA 16341-0329 | Oil & Gas Operator | Dec 2020 |
| 337 | Old Pine Energy Corporation | Austin, TX 78755 | Oil Operator | Jun 2026 |
| 338 | Old South Oil LLC | Marietta, OH 45750 | Oil & Gas Operator | Dec 2025 |
| 339 | Old Timers Oil | Saratoga, TX 77585 | Oil & Gas Operator | Nov 2014 |
| 340 | Old Timers Oil Company, LLC | Saratoga, TX 77585 | Oil Operator | Aug 2022 |
| 341 | Old Timers Oil No. 2 Incorporated | Saratoga, TX 77585 | Oil Operator | Jun 2026 |
| 342 | Old Union Operating Company | Austin, TX 78716 | Oil & Gas Operator | Jun 2026 |
| 343 | Olde Chautauqua Farms | Portland, NY 14769 | Gas Operator | Dec 2024 |
| 344 | Older Than Dirt LLC | Weatherford, TX 76086 | Oil & Gas Operator | Jun 2026 |
| 345 | Oldfield Acquisition LP | Austin, TX 78733 | Oil & Gas Operator | Sep 1996 |
| 346 | Oldfield Operating Corporation | | Oil Operator | Dec 2013 |
| 347 | Olds Oil Company | Casper, WY 82604 | Oil Operator | Jun 1988 |
| 348 | Ole Chaparral On Possum Kingdom | Wichita Falls, TX 76307 | Oil & Gas Operator | Dec 1997 |
| 349 | Ole Crow Operating | Wichita Falls, TX 76308 | Oil Operator | Mar 2026 |
| 350 | Ole Miss Oil & Gas Inc | New Bremen, OH 45689 | Oil & Gas Operator | Dec 1985 |
| 351 | Ole, Inc. | Graham, TX 76450 | Oil & Gas Operator | Aug 2003 |
| 352 | Oleksandr Antonyuk | | Oil & Gas Operator | Dec 2018 |
| 353 | Olenox Corp. | Katy, TX 77449 | Oil Operator | Jun 2026 |
| 354 | Oleum Energy LLC | Georgetown, TX 78628 | Oil & Gas Operator | Feb 2026 |
| 355 | Oleum Exploration, LLC | Brodheadsvill, PA 18322 | Oil & Gas Operator | Mar 2023 |
| 356 | Oleum Inc | | Oil & Gas Operator | Mar 1996 |
| 357 | Oleum Incorporat | | Gas Operator | Dec 1990 |
| 358 | Oleum Operating Company, L.C. | Longview, TX 75606 | Oil & Gas Operator | Dec 2021 |
| 359 | Oleum Operating La LLC | | Oil & Gas Operator | Nov 2022 |
| 360 | Oleum Partners, LLC | Frisco, TX 75034 | Oil & Gas Operator | Jun 2026 |
| 361 | Olia Oil & Gas Services LLC | Cedar Park, TX 78630-2049 | Oil & Gas Operator | Aug 2022 |
| 362 | Oliditt Corporation | | Gas Operator | Mar 2022 |
| 363 | Oliditt Production Inc. | | Gas Operator | Sep 1998 |
| 364 | Olifant Energy LLC | Tulsa, OK 74119 | Oil & Gas Operator | Jun 2019 |
| 365 | Olipdp II, LLC | Lafayette, LA 70503 | Oil & Gas Operator | Jun 2026 |
| 366 | Oliphant Gertrud | Holdenville, OK 74848-0849 | Oil Operator | Aug 2013 |
| 367 | Oliphant Operating Co,LTD,LLC | | Oil Operator | Apr 2026 |
| 368 | Olive Oil & Gas Inc | Jennings, OK 74052 | Oil & Gas Operator | Jan 2026 |
| 369 | Oliver Oil & Gas | Midland, TX 79702 | Oil & Gas Operator | Jun 2026 |
| 370 | Olmos Partners, Inc. | Boerne, TX 78006 | Oil & Gas Operator | Jun 2026 |
| 371 | Olney Oil LLC | Boerne, TX 78006 | Oil Operator | Jun 2026 |
| 372 | Olney Properties, Inc. | Wichita Falls, TX 76307 | Oil & Gas Operator | Sep 2003 |
| 373 | Olsan Brothers, Inc. | Dallas, TX 75251 | Oil & Gas Operator | Apr 1994 |
| 374 | Olsen Energy Inc. | Boerne, TX 78006 | Oil & Gas Operator | Jun 2026 |
| 375 | Olson D Producti | | Oil Operator | Jul 1994 |
| 376 | Olson, S. A. | Kenedy, TX 78119 | Oil & Gas Operator | Aug 1994 |
| 377 | Olson-Anderson Lease | | Oil Operator | Mar 2018 |
| 378 | Oltex Production | Idalou, TX 79329 | Oil & Gas Operator | Jun 2026 |
| 379 | Olympia Oil Inc | Edmond, OK 73083-7088 | Oil & Gas Operator | Jul 2020 |
| 380 | Olympic Exploration & Production Co | Norman, OK 73072 | Oil Operator | May 2003 |
| 381 | Olympic Exploration Inc | Denver, CO 80202 | Oil & Gas Operator | Jul 2004 |
| 382 | Olympic Natural Gas Company | | Oil & Gas Operator | Oct 1990 |
| 383 | Olympic Oil & Gas Co., Inc. | | Oil Operator | Apr 1991 |
| 384 | Olympic Oil, Inc. | Hutchinson, KS 67502 | Oil & Gas Operator | Feb 2026 |
| 385 | Olympic Petroleu | | Oil & Gas Operator | Jul 1993 |
| 386 | Olympic Petroleum Co | Tulsa, OK 74114 | Oil & Gas Operator | Jun 1998 |
| 387 | Olympic Production Company | | Oil & Gas Operator | Jul 1994 |
| 388 | Olympic Resources Co | Wooster, OH 44691 | Oil & Gas Operator | Dec 2000 |
| 389 | Olympus Energy, LLC | Canonsburg, PA 15317-7536 | Gas Operator | Jun 2025 |
| 390 | Olympus Prop Inc | Franklin Park, IL 60131-1418 | Oil & Gas Operator | Dec 1997 |
| 391 | Omac Oil & Gas Co | Newark, OH 43055 | Oil & Gas Operator | Dec 1990 |
| 392 | Omar Operating Company | Midland, TX 79702 | Oil & Gas Operator | Sep 2013 |
| 393 | Omber Petroleum & Energy Corp. | New Braunfels, TX 78131 | Oil & Gas Operator | Jun 2026 |
| 394 | Omeara Bros. | | Oil & Gas Operator | Oct 1982 |
| 395 | Omega Alpha Resources, LLC | Dallas, TX 75219 | Oil Operator | Jun 2026 |
| 396 | Omega Energy Corp. | Graham, TX 76450 | Oil & Gas Operator | Jun 2026 |
| 397 | Omega Energy Incorporated | | Oil & Gas Operator | May 1998 |
| 398 | Omega Energy, LLC | Corpus Christ, TX 78418 | Oil & Gas Operator | Jun 2026 |
| 399 | Omega Enterprise | | Oil Operator | Sep 1990 |
| 400 | Omega Oil Corp | Lafayette, IN 47903 | Oil & Gas Operator | Dec 1987 |