A low-power CMOS RC relaxation oscillator supporting reconfigurable frequencies is presented for timekeeping in low-power Internet of Things (IoT) devices. The oscillator employs a Wheatstone Bridge (WB) scheme to relax the constraint between reference current and reference resistance existing in prior arts. In addition, adjusting the resistor ratios in WB supports reconfigurable frequencies and allows the use of smaller resistors for lower frequencies, saving chip area. Measurements of the prototype in 180nm CMOS show an average temperature coefficient down to 9.6ppm/°C at nominal 67kHz across -20°C to 100°C, with an energy efficiency of 0.84pJ/cycle and an active area of 0.145mm2. Oscillations at 17kHz and 154kHz are also demonstrated by tuning the resistor ratio of WB.