'z' means 'from' like 'jestem z Polski' 'I am from Poland', 'od' means 'from' but in the other contexts like 'robię to od dwóch godzin' 'I am doing it from two hours', 'mieszkam tu od pięciu lat' 'I have lived here since five ears' so here 'od' means 'since'. In sentences like 'I am coming back from church' you should say in Polish 'Wracam z kościoła' but for instance 'I am coming back from my friend' 'Wracam od kolegi'. So when would like to say 'from' which is related with place you should use 'z' and when 'from' is related with person, use 'od'. Hope I have helped a little. Best wishes