Parity-time (PT) symmetry, an interesting concept originally introduced in quantum mechanics, has been extended to many other branches of physics. Besides its demonstrations in optics and electronics, the study of PT-symmetric mechanics is growing rapidly. To date, most PT-symmetric mechanical systems have relatively large size and low operating frequency, which limits their applications in high-speed sensing and signal processing. Here, we propose a PT-symmetric mechanical system with gain/loss provided by the cavity optomechanical effect, which can overcome those limitations. We theoretically analyze and numerically simulate the optical control of PT-symmetric optomechanical systems that consist of two or more mechanical resonators. We find the property of unidirectional reflection in these systems, which may pave the way for the study of topological acoustics and phononic signal processing.