  We present a new optical spectroscopic study of the O-type binary HD 165052
based on high- and intermediate-resolution CCD observations. We re-investigated
the spectral classification of the binary components, obtaining spectral types
of O6.5 V and O7.5 V for the primary and secondary, respectively, finding that
both stars display weak C {\sc iii} $\lambda5696$ emission in their spectra. We
also determined a radial-velocity orbit for HD 165052 with a period of 2.95510
$\pm$ 0.00001 d, and semiamplitudes of 94.8 and 104.7 $\pm$ 0.5 km s$^{-1}$,
resulting in a mass ratio $Q = 0.9$. From a comparison with previous
radial-velocity determinations, we found evidence of apsidal motion in the
system. Several signatures of wind-wind collision, such as phase-locked
variability of the X-ray flux and the Struve-Sahade effect, are also
considered. It was also found that the reddening in the region should be
normal, in contrast with previous determinations.
