Antennas have been used for more than a century to control the emission and collection of radio and microwave radiation. An optical analogue is of great interest as it will allow unique control of absorption and emission at the nanometre scale. Despite the intense recent research on optical antennas, one of the main functions of traditional antennas, the directing of radiation, remains a challenge at optical frequencies. Here we experimentally demonstrate control of the emission direction of individual molecules by reversible coupling to an optical monopole antenna. We show how the angular emission of the coupled system is determined by the dominant antenna mode--that is, the antenna design--regardless of molecular orientation. This result reveals the role of the plasmon mode in the emission process and provides a clear guideline how to exploit the large available library of radio antennas to direct emission in nano-optical microscopy, spectroscopy and light-emitting devices, including single-photon sources.