The spelling of j letter comes from Latin Language and althought you are right and ojala is a word of Arabic origin, no every word with a -j- comes from Arabic. Reloj, and frijoles are words wich come from the old Catalan. In Spanish are lot of words from Arabic, most of them are the words wich start with the prefix al. for example alhambra, almohada, alcalde.... They are too, some wich start with -j-, like jabalí, or jarabe, but not every Spanish word with a " j", comes from Arabic origen