This paper presents an RC-based relaxation oscillator with low temperature coefficient, high power efficiency and fast start-up time for energy-constrained applications. Its event-driven architecture and charge reuse technique reduce the duty cycle of power-hungry blocks, saving power while posing little performance penalty. Implemented in a 0.18-μm CMOS process, the 180-kHz relaxation oscillator exhibits a frequency deviation of ±0.26% against temperature (-40 to 125 °C) from Monte-Carlo simulation, leading to a low temperature coefficient of 32 ppm/°C. The power consumption is 163 nW, resulting in power efficiency of 0.9 nW/kHz.