Single organic dye molecules have been studied for almost three decades. A majority of experiments are now conducted under ambient conditions and aim towards material science and micro-biology. The experiments under cryogenic conditions are often based on highly rigid polycyclic hydrocarbons, which have excellent fluorescing properties when they are operated at temperatures below 2 K. Their photonic advantages integrate their high flux, their narrow-band nature and their tunability over the entire visible spectrum. Therefore the use of organic molecules as efficient and narrow-band single photon sources facilitated a number of quantum optical advances in the past years. The results mostly cover quantum sensing, the formation of quantum hybrid systems and the optical combination of multiple photons in all-optical experiments. Here we present all three fields and outline our own experiments in the combination of single molecule studies, atomic spectroscopy and entanglement generation. The brightness and the narrow-band nature of the molecules are outlined and a delayed-choice quantum eraser is presented. The mode-mixing of two photons on a beam-splitter allows for the generation of a degenerate photon pair, which is resonant to the sodium D<sub>2</sub>-line. Furthermore, its spectral width matches roughly to the natural linewidth of an isolated sodium atom, such that further experiments are feasible. The entanglement of the post-selected photon pair is testified by a violation of Bell's inequality. Even the raw collected clicks violate Bell's inequality by more than two standard deviations.