  O and early B stars are at the apex of galactic ecology, but in the Milky
Way, only a minority of them may yet have been identified. We present the
results of a pilot study to select and parametrise OB star candidates in the
Southern Galactic plane, down to a limiting magnitude of $g=20$. A 2
square-degree field capturing the Carina Arm around the young massive star
cluster, Westerlund 2, is examined. The confirmed OB stars in this cluster are
used to validate our identification method, based on selection from the $(u-g,
g-r)$ diagram for the region. Our Markov Chain Monte Carlo fitting method
combines VPHAS+ $u, g, r, i$ with published $J, H, K$ photometry in order to
derive posterior probability distributions of the stellar parameters $\log(\rm
T_{\rm eff})$ and distance modulus, together with the reddening parameters
$A_0$ and $R_V$. The stellar parameters are sufficient to confirm OB status
while the reddening parameters are determined to a precision of
$\sigma(A_0)\sim0.09$ and $\sigma(R_V)\sim0.08$. There are 489 objects that fit
well as new OB candidates, earlier than $\sim$B2. This total includes 74
probable massive O stars, 5 likely blue supergiants and 32 reddened subdwarfs.
This increases the number of previously known and candidate OB stars in the
region by nearly a factor of 10. Most of the new objects are likely to be at
distances between 3 and 6 kpc. We have confirmed the results of previous
studies that, at these longer distances, these sight lines require non-standard
reddening laws with $3.5<R_V<4$.
