Excavator: Dead wood. The females construct the nests in a variety of existing cavities in dead wood (Banaszak & Romasenko, 1989), a finding not corroborated by Müller et al. (2019.) The cell partitions and nest plug are made of chewed leaves (Benoist 1931, Westrich 1989, Banaszak and Romasenko 2001, Amiet et al. 2004)
Müller et al. (2019) reports finding nests were in pieces of Larix decidua bark, which lay in grassy vegetation on ground exposed to the sun. The bark fragments measured 13-73cm in length, 8.5-34 cm in width and 3-8 cm in thickness. Eight of the bark fragments contained one nest, four fragments two nests and one fragment contained three nests.
The nests studied by Müller et al. (2019) were all excavated by the female bees with no indication that they re-used existing burrows created by other insects or their larvae. The nesting burrows measured 6–7 mm in diameter. They were neither lined with glandular secretions nor with leaf mastic except for several small areas of few mm which were covered with a thin layer of leaf mastic, which was probably applied to smooth out irregularities or to fill small cracks.
The circular nest entrances had a diameter of 6.5-7mm. Three types of partitions were recorded in the nests of O. nigriventris: i) partitions that separate the brood cells from each other; ii) partitions that seal the outermost brood cell towards the nest entrance or a finger towards its mouth, and iii) partitions that close the nest at the entrance.
DNA metabarcoding revealed that the plugs of 5 of the nests were constructed from masticated leaves of Potentilla sp. (Rosaceae) and Helianthemum nummularium (Cistaceae). In addition, a small proportion of the plugs contained Lotus corniculatus (Fabaceae).