The invention relates to an optical sensor comprising an optical waveguide (1) and a light-sensitive element (4) for detecting light coupled out of the waveguide (1) and also various ring resonators (2), the ring resonators (2) being coupled optically to the mentioned waveguide (1) and, with the exception of at most one of the ring resonators (2), each having a means (5) for adjusting resonance frequencies of the respective ring resonator (2) and/or of the coupling between the ring resonator (2) and the waveguide (1) and at least two of the ring resonators (2) having different optical lengths in an initial state and being disposed for having their resonance frequencies influenced by means of different variables to be measured which are specific for each of these ring resonators (2). The invention refers furthermore to a method which can be implemented with a sensor of this type for detecting molecules of at least one substance.