Try not to feel too bad about it, no matter how you lost them. Every person who raises birds loses some at one point or another, no matter how careful they are. An old neighbor of ours had a $3k coop and chicken run that he paid a contractor to build, complete with cement floors and ten-foot walls so his pet Millefleur and Golden Sebright banties could have a clean run to fly around in, and he still lost birds regularly.
From the sound of it, that owl has been near humans a very long time. Owls are one predator we never had to worry about, but I know people who have lost a lot to them and they are very smart and very persistent once they find captive food. If the owl gets into the coop again, I would suggest keeping it in the coop, moving the chicks out to a safe box, and then calling animal control or the local Game Warden. They can remove the owl and take it to a wildlife rehabber who will release it far from people when it's healthy.
For us, raccoons and possums made up for the lack of owls. We had a coon for years who could even open doors and gates, so we constantly had to think up new ways to latch the cages and fences to keep him from having his captive midnight snack. The only latch he never got open was a bolt snap, but it took us forever to figure that out!