Biblical Justification for Oral Tradition

Catholics look to the two sources by which they believe the deposit of faith has been passed down—namely, written Scripture and oral Tradition. My evangelical Tradition has taught that this was not biblical ,  yet here is the biblical justification: 

2 Thess. 2:15 "So then, brothers and sisters, stand firm and hold fast to the teachings we passed on to you, whether by word of mouth or by letter."