"What is the secret to getting dipping chocolate to harden? I melted semisweet morsels with oil and the chocolate did not harden."
There is not much of a secret or trick to dipping something in chocolate and getting it to harden, actually. Simply melt semisweet chocolate by itself or with a little cream or butter. Dip, then refrigerate. When the chocolate is cooled, it hardens. (Adding oil to the chocolate was your downfall.)
See our How to Melt Chocolate video for more tips.