You have two choices, you can perform an FFT after the simulation, in which case the FFT will not be synchronous with the oscillation. In this case the period of the FFT will not match that of the oscillator. You can reduce the artifacts of the period mismatch by simulating many cycles while applying windowing.

Or, you can use SpectreRF.  It computes the periodic steady state analysis, and so it directly computes the period.  It can compute the spectrum without the artifacts.

-Ken