  Massive O and early B stars are important markers of recent star formation
and exert a significant influence on their environments during their short
lives via photoionization and winds and when they explode as supernovae. In the
Milky Way they can be detected at great distances but often lie behind large
dust columns, making detection at short wavelengths difficult. In this study
the use of the less extinguished far-red spectrum (8400 -- 8800 \AA ) for
radial velocity measurement is examined. Results are reported for a sample of
164 confirmed OB stars within a 2-degree field positioned on the Carina Arm.
Most stars are at distances between 3 and 6 kpc, and Westerlund 2 is at the
field edge. The measured radial velocities have errors concentrated in the
3--10 km s$^{-1}$ range, with a systematic uncertainty of 2--3 km s$^{-1}$.
These are combined with Gaia-mission astrometry to allow full space motions to
be constructed. Up to 22 stars are likely to be runaways although 8 of them are
as likely to be interloping (so far undetected) binaries. The mean azimuthal
motion of the sample fits in with recent measurements of Galactic disk
rotation. In the Galactocentric radial direction the mean motion indicates
modest infall at a speed of $\sim$ 10 km s$^{-1}$. This experiment shows that
weak Paschen lines in the far-red can yield credible radial velocity
determination, offering the prospect of exploring OB-star kinematics over much
more of the Galactic disk than has hitherto been possible.
