Any ideas on subgenus? There are a lot of them...12 listed on pp. 2031-2050 of Krombein et al(1), though 3 are monotypic. And it seems there may have been some changes in the subgeneric circumscription since that 1979 tome...as the info page indicates only 9 subgenera. Also, I'm sure new species have been described since "The Red Book"(1) (e.g. 3 here).

I tried to research things a bit...didn't make much progress with the ID for my post, but I added 7 print references to the info page. Among CA Osmia posts on BugGuide, the following looked closest to me (with somewhat golden sheen on head & thorax; white tuft of hairs on face; pale yellow hairs on thorax; and somewhat reddish distitarsi):

       

But I don't know how diagnostic any of those characters may be, and there are a *lot* of species!

Maybe John A. of another expert will eventually be able to help get this a bit further...it's a neat looking bee!

I searched all the male Osmia images on Laurence Packard's photo page. I didn't see any particularly good matches...but the closest seemed to be O. nemoris (image here), though the head & thorax weren't very "golden-bronzish", and the hind legs seemed too swollen.

Nevertheless, I looked up Sandhouse's original 1924 description of the male O. nemoris, and it actually seemed to fit fairly well.

Searching the web, I found O. nemoris was placed in the monotypic subgenus Mystacosmia ("mystac" for mustache), erected by Snelling in 1967 (paper here). At first pass, his detailed descriptions of the male in that paper seemed to me less compatible with my post than Sandhouse's...the passages quoted in green below are Snelling's:

a) "medial flagellar segments no more than 1.5 times as long as wide"...mine has ratio 1.66, but Snelling's key break separating Mystacosmia from Euthosmia uses ratios values of 1.5 and 1.9, respectively;
b) "inner eye margins converging anteriorly" (cf. Fig 1e here)...doesn't appear so in my photos, but perhaps it does in Jim Moore's image here, which I'm thinking is the same species as mine;
c) "with head in dorsal view, all ocelli anterior to line drawn between posterior margins of eyes"...just barely true in my post.
On further reflection, given biological variation and the difficulties of interpreting (let alone formulating) separating characters, it seemed that perhaps the discrepancies above were within an "acceptable margin of uncertainty" not uncommon in trying to match specimens to keys and descriptions?

In fact, cross-referencing with the keys in Hurd & Michener(1)(1955) and Michener(2)(2000)...I see no inconsistency with subgenus Mystacosmia here. In particular, scrutinizing the key to subgenera of Osmia of the Western Hemisphere in Michener(2), it seems the only unclear choice occurs at his couplet 4, since:

1) the parapsidal line appears to be punctiform rather than linear...compare the full-size version of my photo above to Figs. 1 & 3 on page 9 of Hurd & Michener(1). (Note: This character is subtle and can be difficult to discern. If I erred here, the alternative is Diceratosmia, whose only CA species listed in Krombein(3) is O. subfasciata);
2) the malar space appears to be virtually absent in the 2nd image of my series; and
3) this is definitely a male...can count the 11 flagellomeres.
Unfortunately, couplet 4 in Michener's key involves sternite characters not visible in my post, so in addition to Mystacosmia a number of other subgenera could be candidates here (Michener's key just rules out subgenera Diceratosmia, Osmia s. str., Cephalosmia, and Euthosmia).

There is currently one BugGuide post placed under O. nemoris, a female:

   

Since Osmia are typically sexually dimorphic, the differences between my male post and the female above don't rule out nemoris (e.g. compare Packard's female and male images). Also of interest, note that on BugGuide O. nemoris has been placed within subgenus Melanosmia, presumably based on the synonymy of Mystacosmia under that genus published in the 2013 paper by Rightmyer, Griswold, & Brady.

At any rate, whether this is O. nemoris or not, I'm still thinking the BugGuide posts thumbnailed in my "Thanks Hadel" comment above all look like the same species. I made a collage of those images, with color/tint adjustments and added it to this series for comparison purposes: