Possibly a bivoltine species; Else & Edwrads (2018) give a flight period from mid April or May to late July, occasionally August. E. Saunders (1877) collected a male in West Sussex in late October (though this may have been either a precocious or a very late emergence). Stanisavljević (2000) gives a flight period of May to August. Amiet et al. (2004) give a flight period in Switzerland from mid-March to early September.
Baldock et al. (in prep.) report flight activity from May to July in the Balearic Islands (Spain).
Tasei (1972) suggests that in France this species is regularly double-brooded. Nests provisioned there in May produce a second brood, both sexes of which fly in July. Eggs laid a little later in June give rise to females which are active in July, but only half the complement of males produced at this time emerge from their nests, the remainder entering adult diapause and not flying until the following spring. Second brood females flying in July, August and September provision cells in which adults emerge from their pupae in late summer, but these also remain in diapause within their cocoons before finally leaving the nest for the first time the following May. Some French females provision nests as late as October and their offspring overwinter as prepupae. Peeters, Raemakers & Smit (1999) suggest that O. caerulescens is also partially bivoltine in the Netherlands.
Parasites and predators Occupied cocoons of the cleptoparasitic wasp Sapyga quinquepunctata (Sapygidae) have been recovered from a nest of this species (Hallett, 1920, 1928). There are British specimens of this wasp in the NMS which were reared from a nest of O. caerulescens.
Westrich (1989) lists the bee Stelis ornatula as a cleptoparasite of this species.