This paper presents an ultra-low power 915 MHz Local Oscillator (LO) with adaptive frequency calibration algorithm for IoT Wake-Up Receivers (WURs). The proposed algorithm can adaptively adjust and minimize the calibration time of each bit oscillator tuning word (OTW) as well as the number of OTWs that require re-calibration after open-loop time. These two features help reducing the average LO power consumption by minimizing the closed-loop frequency calibration time. The duty cycle of the closed-loop time can also be adjusted flexibly to further reduce the average LO power consumption at a given frequency stability requirement. Fabricated in 40-nm CMOS, this proposed algorithm reduces the frequency calibration time from 108 μs to 42 μs, achieving a time reduction of 61%. An average LO power consumption of 93 μW at a 5% closed-loop time duty cycle is measured with a frequency accuracy of 120 ppm.