Only 11 O-type stars have been confirmed to possess large-scale organized magnetic fields. The presence of a '600 G longitudinal magnetic field in the O9.7 V star HD 54879 with a lower limit of the dipole strength of 1/42 kG was discovered a few years ago in the framework of the ESO large program 'B-fields in OB stars'. Our FORS 2 spectropolarimetric observations from 2017 October 4 to 2018 February 21 reveal the presence of short- and long-term spectral variability and a gradual magnetic field decrease from about '300 G down to about '90 G. Surprisingly, we discover on the night of 2018 February 17 a sudden, short-term increase of the magnetic field strength and measure a longitudinal magnetic field of '833 G. The inspection of the FORS 2 spectrum acquired during the observed magnetic field increase indicates a very strong change in spectral appearance with a significantly lower photospheric temperature and a decrease of the radial velocity by several 10 km s '1. Different scenarios are discussed in an attempt to interpret our observations. Our FORS 2 radial velocity measurements indicate that HD 54879 is a member of a long-period binary.