  High-quality frequency references are the cornerstones in position,
navigation and timing applications of both scientific and commercial domains.
Optomechanical oscillators, with direct coupling to continuous-wave light and
non-material-limited f Q product, are long regarded as a potential platform for
frequency reference in radio-frequency-photonic architectures. However, one
major challenge is the compatibility with standard CMOS fabrication processes
while maintaining optomechanical high quality performance. Here we demonstrate
the monolithic integration of photonic crystal optomechanical oscillators and
on-chip high speed Ge detectors based on the silicon CMOS platform. With the
generation of both high harmonics (up to 59th order) and subharmonics (down to
1/4), our chipset provides multiple frequency tones for applications in both
frequency multipliers and dividers. The phase noise is measured down to -125
dBc/Hz at 10 kHz offset at ~ 400 {\mu}W dropped-in powers, one of the lowest
noise optomechanical oscillators to date and in room-temperature and
atmospheric non-vacuum operating conditions. These characteristics enable
optomechanical oscillators as a frequency reference platform for
radio-frequency-photonic information processing.
