This study presents a measurement-based methodology to efficiently design a harmonic-loaded oscillator at a targeted frequency using the novel multi-harmonic real-time active load-pull (RT-ALP) technique. Owing to the speed of the RT-ALP technique the design procedure is quasi-interactive. Simulations were first used to design a stable negative resistance around 2.5 GHz using series feedback with a variable tapped capacitor to tune the negative resistance in the desired frequency band. The device lines were then measured for characterising the negative-resistance device versus input power at 2.5 GHz. Using RT-ALP the optimal second- and third-harmonic load impedances that provide the maximum output power were then determined. Two different load circuits were then designed and implemented to approach the optimal multi-harmonic load impedances and realise a stand-alone oscillator. In accordance with the Kurokawa theory, a reasonable agreement is obtained in terms of output power and frequency of oscillation between the stand-alone oscillator built and the multi-harmonic load-pull measurements demonstrating the viability of the proposed method. To our knowledge, this is the first fabricated oscillator designed with experimental active load-pull to consider third-harmonic tuning.