I probably do have the specimen, somewhere. Once I find it I will soften the bee and re-pose it for a clear ventral view. After it is dry again I will try to photograph some nice ventral views and upload the best one.

Thank you for taking the time to investigate these specimens further!

That would be great, Jim! I skimmed through the thumbnails for all the BG Osmia posts (>1300!) looking for ventral views...and I found just 5!

                     

Unfortunately, the first one isn't well-lit enough to discern various key characters, and the last two are oblique rather than straight-on views (and dimly-lit underneath as well).

Considering 3 out of 14 couplets in Michener's key emphasis characters of S2-S4...and many authors also emphasize sternite characters in species descriptions...it would definitely be nice to have more clear ventral images. In particular some "straight-on" as well as "oblique-from-behind" close-ups of sternites where the shape of the margins and general flatness vs. convexity are discernible (e.g. for S2-S4 and the terminal sternites).

I wish you luck in finding the specimen, preparing it, and getting nice photos!

PS: I always enjoy your posts when I run into them :-)

I hope these 2 extra photos are clear enough views of the sternite features in question. The ventral view of this specimen sure looks different from the 2 more clear specimens shown above; but I suppose this is due to the deceased & dried nature of the specimen. I sure appreciate the time and effort that you are putting in to this.

And now, I wish you success as you pursue Michener's key further!

And the response is looong!! It's posted under the 1st image in the series below, where I added labels to your last two images here to facilitate pointing out various details of the analysis:

         

I wanted to record/summarize some of what I learned about Osmia motivated by our interchange...though I wonder how many (or few!) readers will have the wherewithal to get through my write-up!! :-) I do such things, in large part, as a future reference for myself...and hope that others will benefit too, if they have the patience and perseverance to read and follow it all!

As far as the ID goes, here's an "executive summary" of two good follow-up items for you do if you can:

1) Examine your specimen to see if T6 (=the 6th dorsal segment of the abdomen) does or does not have the very wide but shallow apical emargination (and much smaller median emargination) shown clearly in this reference image. If it does...you've most likely got Osmia (Diceratosmia) subfasciata. If it does not, then...
2) Check your specimen to see if the malar space is shorter than the width of the scape (=1st antennal segment). If not, you've got something in the very speciose subgenus Osmia (sensu stricto). But if the apical margin of T6 is is different from the reference image above (in particular, if it's "produced in the middle and truncate at its extreme apex"); and if the malar space is smaller than the scape...then it's extremely likely you have Osmia nemoris here.

Thank you Aaron for a great follow up response! Not to long a read for me!

I have added 3 extra photos of details suggested in your response:

1. The specimen is indeed a male based on the presence of 11 antennal flagellomeres; as illustrated in the newly added photo.

2. I was not able to get a clear photo of the hind coxae. But, I was able to view the hind coxae with my small microscope with fairly good detail. I could not discern any evidence of a 'strong longitudinal carina' as illustrated in the links that you provided.

3. I have added a dorsal photo view of the abdominal tergites. If I am understanding the keys correctly concerning O. subfasciata, then I would be inclined to say that my specimen is not O. subfasciata.

4. Photographing the Malar Space was a bit difficult; I have added a ventral view. The narrowest part of the Malar Space seems very thin, certainly thiner than the width of the antennal scape.

So again, walking through Michener's key: seems Diceratosmia is out; and the small malar space eliminates Osmia sensu stricto too. Beyond there, choices from Michener's key seem pretty clear until couplet 6. At that couplet, the medial antennal segment dimensions would seem to make Mytacosmia the better choice. And while I'm not quite sure on how well "T6 prolonged medially over T7" fits (kinda well?)...I'd say the posterior margin of T6 is not "evenly convex", but rather widely & shallowly emarginate.

So overall it seems to me Mystacosmia (and thus O. nemoris) is the best conclusion from Michener's key (and other considerations mentioned previously). I will try to look at specimens at the Essig Museum and/or Cal Academy in the coming weeks. And hopefully some of our more experienced bee workers on BugGuide will also eventually consider the many diagnostic characters illustrated in your photos here, and be able to confirm (or correct) that putative ID.