Massive OB stars are critical to the ecology of galaxies, and yet our knowledge of OB stars in the Milky Way, fainter than $V sim 12$, remains patchy. Data from the VST Photometric H$alpha$ Survey (VPHAS+) permit the construction of the first deep catalogues of blue excess-selected OB stars, without neglecting the stellar field. A total of 14900 candidates with 2MASS cross-matches are blue-selected from a 42 square-degree region in the Galactic Plane, capturing the Carina Arm over the Galactic longitude range $282^{circ} lesssim ell lesssim 293^{circ}$. Spectral energy distribution fitting is performed on these candidates combined VPHAS+ $u,g,r,i$ and 2MASS $J,H,K$ magnitudes. This delivers: effective temperature constraints, statistically separating O from early-B stars; high-quality extinction parameters, $A_0$ and $R_V$ (random errors typically $< 0.1$). The high-confidence O-B2 candidates number 5915 and a further 5170 fit to later B spectral type. Spectroscopy of 276 of the former confirms 97% of them. The fraction of emission line stars among all candidate B stars is 7--8% . Greyer ($R_V > 3.5$) extinction laws are ubiquitous in the region, over the distance range 2.5--3 kpc to $sim$10~kpc. Near prominent massive clusters, $R_V$ tends to rise, with particularly large and chaotic excursions to $R_V sim 5$ seen in the Carina Nebula. The data reveal a hitherto unnoticed association of 108 O-B2 stars around the O5If$+$ star LSS 2063 ($ell = 289.77^{circ}$, $b = -1.22^{circ}$). Treating the OB star scale-height as a constant within the thin disk, we find an orderly mean relation between extinction ($A_0$) and distance in the Galactic longitude range, $287.6^{circ} < ell < 293.5^{circ}$, and infer the subtle onset of thin-disk warping. A halo around NGC 3603, roughly a degree in diameter, of $sim$500 O-B2 stars with $4 < A_0 (rm{mag}) < 7$ is noted.
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