Like all bees, Osmia bicornis determines sex by fertilisation: a fertilised egg becomes a daughter, an unfertilised one a son. The female stores sperm and releases it selectively, which lets her assign sex cell by cell as she works down a tube.
She provisions from the blind end outwards. The innermost cells receive larger pollen loaves and fertilised eggs, producing daughters, which are the larger and costlier sex. Cells nearer the entrance get smaller provisions and unfertilised eggs, producing males. A woodpecker, parasitic wasp or bird reaching into the tube encounters sons first. Males also emerge earlier in spring, which the arrangement conveniently allows since they are closest to the exit.
The emerging order is why bee hotels should not be cleaned by tipping cocoons out of order, and why short or wide tubes skew colonies towards males. The frequent claim that a single mason bee outperforms hundreds of honey bees comes from per-visit pollen deposition on orchard blossom rather than from any measure of colony output, and the two figures are not comparable.