The invention provides an oscillator circuit has a reconfigurable oscillator amplifier. The reconfigurable oscillator amplifier is used to be coupled to a resonant circuit in parallel. The reconfigurable oscillator amplifier supports different circuit configurations for different operation modes, respectively. The reconfigurable oscillator amplifier has at least one circuit component shared by the different circuit configurations. The reconfigurable oscillator amplifier employs one of the different circuit configurations under one of the different operation modes. The invention also provides a method for configuring the oscillator amplifier that is coupled in parallel to a resonant circuit. Because oscillating amplifiers with hardware sharing can reduce the number of circuit elements required to implement different circuit configurations, the reconfigurable oscillator amplifier can be implemented with smaller chip area, thereby reducing manufacturing costs.