In microwave negative resistance oscillators, the RF transistor presents impedance with a negative real part at either of its input or output ports. According to the conventional theory of microwave negative resistance oscillators, in order to sustain oscillation and optimize the output power of the circuit, the magnitude of the negative real part of the input/output impedance should be maximized. This paper discusses the effect of the circuit’s load impedance on the input negative resistance and other oscillator performance characteristics in common base microwave oscillators. New closed-form relations for the optimum load impedance that maximizes the magnitude of the input negative resistance have been derived analytically in terms of the Z-parameters of the RF transistor. Furthermore, nonlinear CAD simulation is carried out to show the deviation of the large-signal optimum load impedance from its small-signal value. It has been shown also that the optimum load impedance for maximum negative input resistance differs considerably from its value required for maximum output power under large-signal conditions. A 1.8 GHz oscillator circuit has been designed and simulated using a typical SiGe hetero-junction bipolar transistor (HBT) to verify the proposed approach of analysis.