Past exam of the mathematics course of the University of Cambridge / 2020 / ii / Paper 2 / 37D / iii / Solution

Since $r>3M$,Relative to the distant observer, the static clock $O_2$ suffers gravitational time dilation. The orbiting clock $O_1$ suffers the same gravitational effect together with the kinematic time dilation caused by its orbital motion, so it accumulates the least proper time.