Take a flight to the Split Airport and then use a Bus service to get to Omis.
There is taxi-van service that can be used from Split and Makarska
There is a ferrie service from Ancona,Italy. Take a ferrie to the Port of Split and than use the bus to get to Omis. There are local boats that run several times a day from Islands of Brac, Hvar, Korcula and Vis. Take them to Split and then use the bus or taxi service to get to Omis.