The large bandwidth, low propagation loss and immunity to eletromagnetic disturbances are major advantages of microwave photonics, whereby optical communication technologies are used in analog signal processing. The local oscillator is the cornerstone of these architectures; here, it consist of an optical carrier with a high purity modulation, a sort of optical clock which can be distributed reliably and easily all over the microwave system. Such a clock can be implemented in an opto-electronic loop involving a carrier which is modulated by a positive electronic feedback. These oscillators challenge in purity the established technology of quartz clock when they are compared in the microwave domain. The performance is ascribed to the long time delay (µs) in the feedback, owing to an optical fiber. CRONOS project is about a radically new approach allowing the miniaturization of the delay line. As the opto-electronic oscillator is based on a km long fiber, integration is impossible. Here, we will develop an opto-electro-mechanical loop with a millimeter-sized acoustic delay line, which indeed provides the required delay owing to the much lower speed of sound as compared to light. This requires a new technology where the interaction between light and sound results into an effective transduction. The heart of the system is an opto-mechanical crystal, allowing both mechanical and optical resonant modes which are spatially overlapped. Sub-micron patterning is here the key enabling the strong confinement (within a few µm) resulting into an effective light-sound interaction. Thus, a very sensitive detection of the displacement is possible, as the quantum limit for mechanical displacement results in 0.1 MHz to 1 MHz frequency shift of the resonance. The signal generated by the movement of the resonator is transduced in the optical domain, detected, amplified and finally re-injected as an acoustic wave propagating with the desired delay through an acoustic circuit before reaching the resonator. To this purpose, CRONOS will use a hybrid technology where a silicon photonic circuit will be associated to a III-V compound semiconductor alloy ensuring the best opto-mechanical performances and enabling the excitation of acoustic wave through the piezo-electric effect. The propagation of the acoustic wave is controlled through phononic band engineering owing to the periodic patterning of the material. Our first free-running optomechanical crystal have already demonstrated a fairly low phase noise (corresponding to a short-term linewidth about 100 Hz). CRONOS will improve this figure drastically, primarily by exploiting an electro-optomechanical loop which will reduce both the intrinsic phase noise and a servo loop to defeat instabilities and environmental perturbations. These challenging goals will be reached owing to the partnership between an academic institution, C2N (Centre de Nanosciences et de Nanotechnologies -UMR 9001, Palaiseau) and an industrial laboratory, TRT (Thales Research & Technology, Palaiseau) with a long track record of fruitful and ongoing collaboration. The consortium benefits from cutting-edge facilities for nanofabrication and an established expertise in semiconductor processing, nanophotonics, acoustic and microwave photonics, opto-electronic oscillators in particular.