To access the hostel from the Seoul Subway, take Line 2 and exit at Hongik University Station 239. Then take the Exit 3. When you are out, keep going straight and take the third street on the left and left again to the small passage. It should not take you more than 3 minutes.
Note: If you come from the airport, you just have to take the Airport Railroad Express (AREX) subway line which will bring you directly to Hongik University. It takes 45 minutes from Incheon Airport and 13 minutes from Kimpo Airport.
OMG Hostel is located at Donggyo-ro 212-26, Mapo-gu, Seoul.
And if you're still not sure about the road to take after that, you can watch this short video below showing you that the hostel is really less than 3 minutes from Hongik University Station.