The paper presents the experimental observation of optomechanically induced transparency (OMIT) in a cavity optomechanical system. OMIT is formally equivalent to electromagnetically induced transparency (EIT) in atomic systems, where a control laser induces a narrow spectral transparency window for a weak probe laser beam. In the optomechanical setup, a control laser tuned to the lower motional sideband of the cavity resonance induces a dipole-like interaction between the optical and mechanical degrees of freedom. Under these conditions, destructive interference of excitation pathways for an intracavity probe field results in a window of transparency when a two-photon resonance condition is met. The power of the control laser determines the width and depth of the probe transparency window. The experiment uses toroidal whispering-gallery-mode microresonators, which feature low effective mass, high mechanical frequency, and low optical dissipation, allowing for reaching the resolved-sideband regime. The results show that the probe power transmission can be significantly altered by the control beam, achieving high contrast and group delays up to several seconds. This work demonstrates the potential of OMIT for delaying, slowing, and storing light pulses in long-lived mechanical excitations of optomechanical systems.The paper presents the experimental observation of optomechanically induced transparency (OMIT) in a cavity optomechanical system. OMIT is formally equivalent to electromagnetically induced transparency (EIT) in atomic systems, where a control laser induces a narrow spectral transparency window for a weak probe laser beam. In the optomechanical setup, a control laser tuned to the lower motional sideband of the cavity resonance induces a dipole-like interaction between the optical and mechanical degrees of freedom. Under these conditions, destructive interference of excitation pathways for an intracavity probe field results in a window of transparency when a two-photon resonance condition is met. The power of the control laser determines the width and depth of the probe transparency window. The experiment uses toroidal whispering-gallery-mode microresonators, which feature low effective mass, high mechanical frequency, and low optical dissipation, allowing for reaching the resolved-sideband regime. The results show that the probe power transmission can be significantly altered by the control beam, achieving high contrast and group delays up to several seconds. This work demonstrates the potential of OMIT for delaying, slowing, and storing light pulses in long-lived mechanical excitations of optomechanical systems.