This link has the most info and some videos showing mobbing behavior. Click through this one if you want more info.

Also discusses nest stealing which is an interesting subject also.

Why Do Crows Attack Owls? (Chasing & Mobbing Behaviors)

Have you ever noticed when you hear a group of crows cawing & making an awful racket in the woods, there’s often an owl at the center of those mobbing birds? It’s a pretty amazing thing to see!

In fact, if you know to listen for these crow mobbing calls, it’s one of the easiest ways to find owls in the woods.

But it also brings up the question of why crows attack them so intensely?

As a general rule, crows attack owls to reduce their risk of predation. Owls do sometimes eat crows and they frequently have overlapping nest habitats which puts them in close proximity.

However, if you watch this behavior for long enough it becomes apparent that crows are expending an incredible amount of energy attacking owls.

It also sometimes seems like they’re putting themselves at risk by coming so close to a dangerous predator, and yet the owls don’t really seem to react all that aggressively.

Mobbing looks like a group of birds all crowding around the danger, calling & even dive-bombing it. If the owl flies away, mobbing crows will often pursue and chase the target in a dramatic aerial battle.

So then why do crows care so much about owls?

Well, it’s a bit of a chicken and egg situation because given the right opportunity an owl certainly would eat a crow!

However, it’s very likely that part of the reason why crows don’t get eaten more often is because this mobbing, chasing & attacking strategy is such an effective defence.

There’s simply no way of knowing how often owls would hunt crows if they weren’t being pre-emptively attacked.