Is ...o 'r blaen ...the same as....cyn?
No, not exactly. You use cyn when you are saying before something e.g "Cyn dod", "Before coming", where as "o'r blaen" is more "Dw i byth wedi bod yma o'r blaen", "I have never been here before".
okay so previously might be better?
Good to see this accepts "cyn" as well.