For BOBs the preferred internal cavity dimensions are 19/64 inch (7.5 millimeters) in diameter by 6 inches (~152 millimeters) in length. These dimensions are highly attractive to nesting females and result in the highest number of female offspring. Smaller diameters and shorter cavity lengths result in more male offspring and fewer offspring altogether. Larger diameters and longer cavity lengths on the other hand require more effort (for building mud walls, and depositing food provisions) and are less attractive to nesting females.
The hornfaced bee is slightly smaller on average than the BOB and prefers correspondingly smaller nest cavities. Current recommendations among some producers in Japan are for internal diameters between 7/32 and 15/64 inch (~5.75 and 6 millimeters), and lengths between 423/32 and 51⁄2 inches (~12 and 14 centimeters).
Other Osmia species likely prefer other tunnel dimensions based on their physical size, but little published information exists.
In addition to cavity dimensions, inter-hole distances are also an important consideration that influences bee health and the time required for nest provisioning. Nests with flat front surfaces—such as drilled blocks or grooved boards—should have inter-hole distances of 3⁄4 inch (19 millimeters) or even higher. This distance will result in easier orientation for female bees that are returning to the nest from foraging trips. Bees that can easily find their nest hole will complete nests more quickly and are less likely to enter the wrong nest holes where they could become contaminated with disease spores. Many commercially available nests tend to have higher hole densities which can be problematic.
Nests consisting of raised or textured orientation patterns, and irregular surfaces (such as bundles of reed cut in various irregular lengths), result in easier orientation for returning foragers, and in these cases the issue of 3⁄4-inch inter-hole distances can be ignored.
Where nests blocks with flat surfaces are used, the front surface should be painted in alternating geometric patterns to help with orientation. Similarly, if bundles of cardboard straws are used, the openings of random straws should be painted in various colors to assist with navigation. Anecdotal evidence suggests that alternating dark colors such as blue and black are most attractive.