8.
Ignore OT, ST and RT, they're not helpful.
Point T is the meet of the perpendicular bisectors of the sides.
The thing about perpendicular bisectors is they're the locus of points equidistant from the endpoints of the segment being bisected. So the place where all three meet must be the point equidistant from all three vertices. So it must be the center of the circle through the vertices. That circle is called the circumcircle and its center the circumcenter.
