NGC 6543 is one of the few planetary nebulae whose X-ray emission has been shown to be extended and to originate from hot interior gas. Using Far Ultraviolet Spectroscopic Explorer observations, we have now detected nebular O VI emission from NGC 6543. Its central star, with an effective temperature of ~50,000 K, is too cool to photoionize O V, so the O VI ions must have been produced by thermal collisions at the interface between the hot interior gas and the cool nebular shell. We modeled the O VI emission incorporating thermal conduction but find that simplistic assumptions for the asymptotic giant branch and fast wind mass-loss rates overproduce X-ray emission and O VI emission. We have therefore adopted the pressure of the interior hot gas for the interface layer and find that expected O VI emission to be comparable to the observations. Based on observations made with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer. FUSE is operated for NASA by The Johns Hopkins University under NASA contract NAS 5-32985.