This paper describes the cooling of a nanomechanical oscillator to its quantum ground state using optical radiation pressure. A patterned silicon nanobeam is designed to support both acoustic and optical resonances, which are coupled via radiation pressure. Starting from a bath temperature of approximately 20 K, the 3.68 GHz nanomechanical mode is cooled to a final phonon mode occupancy of \(\bar{n} = 0.85 \pm 0.04\). The system consists of an integrated optical and nanomechanical resonator on a silicon-on-insulator microchip, with an external acoustic radiation shield to minimize mechanical damping. The cooling process involves using a tunable laser to optically cool and transduce the mechanical motion, with the device placed in a continuous-flow helium cryostat to pre-cool it to a temperature of about 20 K. The cooling efficiency is demonstrated through measurements of the mechanical noise power spectrum and the optically measured mechanical mode bath temperature. The results show that the system can be cooled to a quantum ground state at an environmental temperature significantly higher than in previous experiments, paving the way for optical control of mesoscale mechanical oscillators in the quantum regime.This paper describes the cooling of a nanomechanical oscillator to its quantum ground state using optical radiation pressure. A patterned silicon nanobeam is designed to support both acoustic and optical resonances, which are coupled via radiation pressure. Starting from a bath temperature of approximately 20 K, the 3.68 GHz nanomechanical mode is cooled to a final phonon mode occupancy of \(\bar{n} = 0.85 \pm 0.04\). The system consists of an integrated optical and nanomechanical resonator on a silicon-on-insulator microchip, with an external acoustic radiation shield to minimize mechanical damping. The cooling process involves using a tunable laser to optically cool and transduce the mechanical motion, with the device placed in a continuous-flow helium cryostat to pre-cool it to a temperature of about 20 K. The cooling efficiency is demonstrated through measurements of the mechanical noise power spectrum and the optically measured mechanical mode bath temperature. The results show that the system can be cooled to a quantum ground state at an environmental temperature significantly higher than in previous experiments, paving the way for optical control of mesoscale mechanical oscillators in the quantum regime.