| Ocean Rover (ship) | Vol.23.No.2.Page 5 |
| O. K. windmill | - see - Challenge O. K. Solid Wheel windmill |
| Ornamental windmills | - see - Comet Windmills Australia - D Pattern 3ft windmill |
| Orr & Sembower steam engine | Vol.21.No.2.Page 3 |
| Osborne, Sarah Anne | Vol.24.No.1.Page 2 |
| Osbourne, W. F. | Vol.20.No.4.Page 1 |