A method of monitoring a substance in a well can include disposing at least one optical electromagnetic sensor and at least one electromagnetic transmitter in the well, and inducing strain in the sensor, the strain being indicative of an electromagnetic parameter of the substance in an annulus between a casing and a wellbore of the well. A system for monitoring a substance in a well can include at least one electromagnetic transmitter, and at least one optical electromagnetic sensor with an optical waveguide extending along a wellbore to a remote location, the sensor being positioned external to a casing in the wellbore.