An oscillator includes a first resonator element, a first circuit element configured to oscillate the first resonator element to generate a first oscillation signal, a first package which is configured to house the first resonator element and the first circuit element, and has a mounting surface and a mounting terminal disposed on the mounting surface, a second resonator element an oscillation frequency of which is controlled based on the first oscillation signal, and a second package which houses the second resonator element and is provided with the second package mounted on the mounting surface of the first package.