  We show that optomechanical systems in the quantum regime can be used to
demonstrate EPR-type quantum entanglement between the optical field and the
mechanical oscillator, via quantum-state steering. Namely, the conditional
quantum state of the mechanical oscillator can be steered into different
quantum states depending the choice made on which quadrature of the out-going
field is to be measured via homodyne detection. More specifically, if quantum
radiation pressure force dominates over thermal force, the oscillator's quantum
state is steerable with a photodetection efficiency as low as 50%, approaching
the ideal limit shown by Wiseman and Gambetta [Phys. Rev. Lett. {\bf 108},
220402 (2012)]. We also show that requirement for steerability is the same as
those for achieving sub-Heisenberg state tomography using the same experimental
setup.
