The official explanation of the name from the city's web site:

Although the name of the city - Opa-locka - sounds Arabic, it is a contraction of the native Americans' name for the area, "Opa-tisha-woka-locka", meaning a dry place in the swamp with trees. Obviously this would not readily fit on a letterhead and be rather difficult to remember. Hence it was shortened to Opa-locka.