| Obsidian Cliffs | |
| Occidental Hotel | |
| Odd Fellows Building, Casper | Stephanie Lowe |
| Oil business, early emigrant trails | Allan Fraser |
| Oil Fields, Five of Wyoming’s historic | Rebecca Hein |
| Oil refining, Wyoming | Tom Mast |
| Oil theft, Wind River Reservation | Marjane Ambler |
| Oil, history of in Wyoming | Phil Roberts |
| Okie, J. B. | Tom Rea |
| Old Faithful geyser, Yellowstone National Park | |
| Old Faithful Inn, Yellowstone National Park | John Clayton |
| Old West Museum | |
| Opal Hub, The | |
| Oregon Basin Oil Field | Rebecca Hein |
| Oregon Buttes | |
| Oregon Buttes, Oregon Trail landmark | WyoHistory.org |
| Oregon Trail Ruts | |
| Oregon Trail Ruts | Wyoming State Historic Preservation Office |
| Oregon Trail-Ferdinand V. Hayden Monument | |
| Osborne, Gov. John E., and Wyoming politics, 1892-1893 | Lori Van Pelt |
| Osborne, John | Wyoming State Archives |
| Ostrom, George, of Sheridan, World War I soldier and artist | Douglas R. Cubbison |
| Outfitting in Wyoming | Brian Beauvais |
| Overland Trail | David Johnson |
| Overland Trail, defense of | Rebecca Hein |
| Overton, Carrie Burton. First African-American Female Student at UW | Kim Viner |
| Oyster Ridge Music Festival | |
| O’Bryan, Lowell, death of on UW campus, 1922 | Phil Roberts |
