In the original anime Shingeki no Kyojin (Attack on Titan), there was an OST playing at 18 minutes and 40 seconds into the sixteenth episode. This OST happened when 104th Cadet Corps were receiving their survey corps cape.
If you are talking about the season 1, I believe the artist is Hiroyuki Sawano. That song is on OST 2, track 6, if I'm not mistaken. The name of the song is kind a complicated (進撃vn-pf20130524巨人) so I will just leave a link.