I saw a local Raven with a golf ball a few years ago. I thought to myself "stupid Crow, thinks it has an egg". That was before I knew that there are no Crows in Sydney, they are all Ravens. I also did not know that Ravens take golf balls, break them open and unwind the internal bands to use as nest material.
To stay on Owl topic, from Wikipedia page on Powerful owls:
Also, they are often victim to and occasionally even injured by heavy mobbing by larger passerines such as currawongs, magpies, and crows and ravens. In one case, a pair of Australian ravens (Corvus coronoides) attacked and killed a powerful owl, likely to defend themselves and their nest.
McNABBI, E. G., KAVANAGH, R. P., & CRAIG, S. A. (2007). FURTHER OBSERVATIONS ON THE BREEDING BIOLOGY OF THE POWERFUL OWL Ninox strenua lN SOUTH-EASTERN AUSTRALIA.