Mobbing of owls by songbirds or crows may benefit the songbirds or crows in three different ways. It may drive an owl away, it may teach inexperienced offspring that an owl is a dangerous predator, or it may warn naive young that a predator is nearby and that caution is needed. Although owls will sometimes flee from a mob of agitated crows, they are just as likely to do nothing. The owls response may depend on the size of the birds that are mobbing it and the potential risk of injury. For example a Great Horned Owl will usually try to escape to a safer, more hidden refuge. Crows will commonly follow a retreating owl, calling loudly, possibly to rally other crows to join in the chase.
At the 38 second mark of this video the owl is struck violently on the back of the head by a diving crow. This causes the owl to seek a safer, possibly more hidden roost.