Three Galactic O-type stars belong to the rare class of Of?p objects: HD 108, HD 191612, and HD 148937. The first two stars show a wealth of phenomena, including strong X-ray emission, light variability, and dramatic periodic spectral variability. We present here the first detailed optical and X-ray study of the third Galactic Of?p star, HD 148937. Spectroscopic monitoring has revealed low-level variability in the Balmer and He II ?4686 lines, but constancy at He I and C III ?4650. The H? line exhibits profile variations at a possible periodicity of ~7 days. Model atmosphere fits yield T eff = 41000 ± 2000 K, log(g) = 4.0 ± 0.1, \dot{M}_sph\lesssim 10^{-7} M sun yr 1, and an overabundance of nitrogen by a factor of 4. At X-ray wavelengths, HD 148937 resembles HD 108 and HD 191612 in having a thermal spectrum dominated by a relatively cool component (kT = 0.2 keV), broad lines (>=1700 km s 1), and an order-of-magnitude overluminosity compared to normal O stars (log[L unabs X/L BOL] ~ 6). Based on observations collected with SMARTS and XMM-Newton, an ESA Science Mission with instruments and contributions directly funded by ESA Member States and the USA (NASA).