Getting this to identification as O. parvum was quite a slog with Bohart & Schlinger's key, until I overexposed the rear view of the postnotum (see last upload) and could make out that the mucro/propodeal process has a flared shape with concave rear edge, and the squamae of the postscutellum have pointed tips along with a shorter submedian lobe as in Bohart & Schlinger's Fig. 67. Their Fig. 85 of O. argenteopilosus has a very similar shape of squamae and process, as well as overlapping range, and the only difference they consider diagnostic is that O. parvus does not have a setose area in the median cell of the forewing - not something that can be seen at regular macro lens magnification.

I'm placing my bets on O. parvusas ID for my photos, because Bohart & Schlinger's final key couplet for O. argenteopilosus mentions that its squamae often have equally long lateral tooth and submedian lobe. However that doesn't seem to be really distinctive: in their Fig. 85 the lobes are clearly shorter than the teeth, and on p.123 they write details about variation in O. argenteopilosus that ranges from short submedian lobes to those that equal the lateral teeth.

Possibly the californian female saved from drowning and then uploaded by kneubauer could be O. parvum too, judging from the closely matching coloration (except that it has yellow foretarsi) and appearance of a two-tipped mucro in the side view photo. The shape of the squamae isn't visible however, and of course the presence/absence of setae in the median cell isn't discernible either.