We report on the detection of magnetic field strength and velocity oscillations in the photosphere of a sunspot umbra. Our analysis is based on the inversion of the full Stokes vector of three Fe I lines at 15650 Å, from which the stratification with optical depth of the different atmospheric parameters has been derived. This allows us to estimate the amplitude of the oscillations and the phase lag between the fluctuations in the line-of-sight velocity and field strength. Our results suggest that the inferred magnetic field oscillations are caused by opacity fluctuations that move upward and downward the region where the spectral lines are sensitive to magnetic fields.