Unless by sheer accident, as ours occurred, it is not lawful to trap an owl. One can only provide stopgaps to protect your chickens from predators...like dogs free ranging with the flock, hides under which the chickens can duck, a good and watchful roo to sound the alarm, no stooping to pick up your birds so often that they grow accustomed to shadows from above, etc. Breed for flighty, wary birds who are watchful when free ranging, keep your survivors and breed from their stock so they will teach their young the same level of wariness, etc.
If penned, one can stretch plastic netting over the penned area, string fishing line, or some folks have had good luck with placing the shiny whirly-gigs around their chicken's living areas.