For owls that are superb.
US Wild Animal Rescue Database: Animal Help Now
International Wildlife Rescues: RescueShelter.com
Australia Rescue Help: WIRES
Germany-Austria-Switzerland-Italy Wild Bird Rescue: wildvogelhilfe.org
If you find an injured owl:
Note your exact location so the owl can be released back where it came from. Contact a licensed wildlife rehabilitation specialist to get correct advice and immediate assistance.
Minimize stress for the owl. If you can catch it, toss a towel or sweater over it and get it in a cardboard box or pet carrier. It should have room to be comfortable but not so much it can panic and injure itself. If you can’t catch it, keep people and animals away until help can come.
Do not give food or water! If you feed them the wrong thing or give them water improperly, you can accidentally kill them. It can also cause problems if they require anesthesia once help arrives, complicating procedures and costing valuable time.
If it is a baby owl, and it looks safe and uninjured, leave it be. Time on the ground is part of their growing up. They can fly to some extent and climb trees. If animals or people are nearby, put it up on a branch so it’s safe. If it’s injured, follow the above advice.
For more detailed help, see the OwlPages Rescue page.
From Ted Smith
Kestrel attacking a Barn Owl, a daily event here in North Norfolk. This was a territorial fight but usually the Kestrel attacks when the Owl has prey. It bullies the Owl into dropping the prey which is then stolen. The Owl was unharmed by the attack and carried on hunting afterwards.
Interesting TIL: Kestrel and Barn Owls fight in the wild!
Owls get hassled a lot by birds of all sizes. They know the owls are dangerous or at least tough competition.
Here's a good post of some Great Horned Owls giving some revenge back to some Bald Eagles for previous harassment like in this post.