Overdensities of candidate PMS sources in the OB1a region surrounding the 25 Ori cluster. The grayscale map is the surface density of photometric candidate PMS sources selected from the CVSO photometric survey based on their location in the PMS locus of the V vs. V − J CMD and detected as variable in the optical bands. Red stars correspond to early-type stars of B1–B2 spectral type, red triangles to B3–B5 stars, and magenta dots to B6–B9 stars. We have highlighted the B2 stars 25 Ori, HR 1833, and HD 35672 with large blue stars.