| ONEOK Eastbound Expansion | ONEOK Inc. | OK | In service 2019 | |
| ONEOK Rockies Midstream Cherry Creek Pipeline-McKenzie & Williams Counties | Oneok Rockies Midstream, LLC | ND | | |
| ONEOK Rockies Midstream Demicks Lake Gas Processing Plant - McKenzie Cnty | Oneok Rockies Midstream, LLC | ND | 2023 | |
| ONEOK Rockies Midstream Lonesome Creek Gas Plant - McKenzie County, ND | Oneok Rockies Midstream, LLC | ND | | |
| ONEOK WesTex Transmission Pipeline Expansion | Oneok Westex Transmission, Inc. | TX | | |
| ONEOK WestTex Expansion | ONEOK Inc. | TX | In service 2019 | |
| ONEOK Westbound Expansion | ONEOK Inc. | OK | In service 2019 | |
| OPEN (Ohio Pipeline Energy Network) Project | Texas Eastern Transmission, LP | OH | 2015 | |
| Oak Creek Delivery Point Project | Mojave Pipeline Company, L.L.C. | CA | Started 2022 | |
| Oak Gate Pipeline (Blue Ridge Interconnect) | Eqt XL Midstream Operating LLC | WV | | |
| Oak Grove Enhancement Project | ANR Pipeline Company | LA | 2024–2025 | |
| Oasis Pipeline Modernization Project | Energy Transfer | TX | In service 2022 | |
| Occidental Chemical coastal use permit P20021469 pipeline | Occidental Chemical Corp | LA | | |
| Occidental Chemical coastal use permit P20040215 pipeline | Occidental Chemical Corp | LA | | |
| Occidental Chemical coastal use permit P20141435 pipeline | Occidental Chemical Corp | LA | | |
| Ocean Express Pipeline (LNG/Bhms | AES Ocean Express LLC | FL | | |
| Odessa to Stanton | | TX | | |
| Odessa-Ector Power Pipeline | Aspen Pipeline LP | TX | In service 2007 | |
| Ogden Compressor Replacement | Northern Natural Gas Company | IA | | |
| Ogden Compressor Replacement Project | Northern Natural Gas Company | IA | In service 2008 | |
| Ohio Gathering Pipeline | Rice Energy | OH | | |
| Ohio Intrastate Gas System A/B OPSB case 96-0710-GA-BLN | Ohio Intrastate Gas | OH | | |
| Ohio Pipeline Project | Sunoco Pipeline L.p. | WV | | |
| Ohio River Crossing Replacement Project | Texas Eastern Transmission, LP | OH | Started 2023 | |
| Ohio Storage Expansion Project | Colubmia Gas Transmission Corp. | OH | | |
| Ohio Valley Connector | EQM Midstream Partners (Equitrans) | WV, OH | 2016 | |
| Ohio Valley Connector Expansion Project | Equitrans, L.P. | OH, PA, WV | In service 2024 | |
| Ohio Valley Hub Line | | IN | In service 2001 | |
| Ohio Valley Midstream 12-inch Ethane Project | Williams Field Services | PA | | |
| Ohio-Louisiana Access Project | Texas Gas Transmission, LLC | LA | In service 2016 | |