  Although people have already artificially formed parity--time
($\mathcal{PT}$) symmetry with gain and loss in a balanced manner, it is still
a defect that the gain is restricted to semi--classical but not full quantum.
Here we propose and analyze a theoretical scheme to realize full quantum
oscillator $\mathcal{PT}$--symmetry. The quantum gain is provided by a
dissipation optical cavity with blue detuned laser field. After adiabatically
eliminating the cavity modes, we give an effective master equation, which is a
more complete quantum description compared with non--Hermitian Hamiltonian, to
reveal the quantum behaviors of such a gain oscillator. This kind of
$\mathcal{PT}$--symmetry can eliminate the dissipation effect in quantum
regime. As examples, we finally apply $\mathcal{PT}$--symmetric oscillators to
enhance optomechanically induced transparency and to preserve oscillator
non--classical state.
