| Title | Author |
|---|---|
| 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 |