This paper proposes an ultra-low power fully on-chip CMOS relaxation oscillator (ROSC) for a real-time clock application. The proposed ROSC employs a compensation circuit of a comparator's non-idealities caused by offset voltage and delay time. The ROSC can generate a stable, and 32-kHz oscillation clock frequency without increasing power dissipation by using a low reference voltage and employing a novel compensation architecture for comparators. Measurement results in a 0.18-μm CMOS process demonstrated that the circuit can generate a stable clock frequency of 32.55 kHz with low power dissipation of 472 nW at 1.8-V power supply. Measured line regulation and temperature coefficient were 1.1%/V and 120 ppm/°C, respectively.