Normally, I would agree with the o-scope method.
Today, ultralow jitter measurements are more acuarately measured using a spec analyzer. Close in phase noise contributes the most jitter.
First thing you should do is determine what type of jitter you want to measure... (Cycle-to-cycle, edge-to-edge, etc...) Setup for measuring is different for each type. Also, do you know if your 1ps jitter spec, is rms or peak-to-peak?