What is the name of the OST playing at 18:40 in Shingeki no Kyojin Episode 16?
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.