An oscillator includes a reference voltage generator configured to generate a reference voltage varying according a change in temperature and an external voltage, a first comparison voltage generator configured to output a first comparison voltage to a first node in response to the reference voltage, a second comparison voltage generator configured to output a second comparison voltage to a second node in response to the reference voltage, a first comparison circuit configured to compare the reference voltage and the first comparison voltage and to generate a first input voltage as a result of the comparison, a second comparison circuit configured to compare the reference voltage and the second comparison voltage and to generate a second input voltage as a result of the comparison, and a clock generator configured to output a clock signal that oscillates in response to the first and second input voltages.