  HD 54879 is the most recently discovered magnetic O-type star. Previous
studies ruled out a rotation period shorter than 7 years, implying that HD
54879 is the second most slowly-rotating known magnetic O-type star. We report
new high-resolution spectropolarimetric measurements of HD 54879, which confirm
that a full stellar rotation cycle has been observed, and derive a rotation
period of $P = 2562^{+63}_{-58}$ d (about 7.02 yr). The radial velocity of HD
54879 has been stable over the last decade of observations. We explore
equivalent widths and longitudinal magnetic fields calculated from lines of
different elements, and conclude the atmosphere of HD 54879 is likely
chemically homogeneous, with no strong evidence for chemical stratification or
lateral abundance nonuniformities. We present the first detailed magnetic map
of the star, with an average surface magnetic field strength of 2954 G, and a
strength for the dipole component of 3939 G. There is a significant amount of
magnetic energy in the quadrupole components of the field (23 percent). Thus,
we find HD 54879 has a strong magnetic field with a significantly complex
topology.
