Link between phase noise and linewidth for a oscillator?
The line width is not 100% cleanly defined, as far as I know. What is common that you call it the "width" of the spectral main peak of an oscillator. "Width" needs to be defined, either in terms of "x % of the energy are within that bandwidth" or "outside of the bandwidth, the PSD is x dB below main peak".

In the first case, it becomes very easy: You just integrate the PSD of your oscillator, as given in dBc/Hz values. 

An often good assumption is that the phase noise itself is white, so that the PSD of the oscillator generally has 1/f² behaviour; use that to fit a curve through the points given in the datasheet. Then integrate until you get e.g. 99.9% of the total energy of the noise.

In the second definiton, you'd simply plot your PSD and find the point where it cuts -80 dB (or whatever) of its main peak.

Notice that neither 99.9% nor -80 dB are something standardized. Whoever is supplying you with linewidth info hopefully defines what they mean.