Those are two pretty different artists you mention. Conner Oberst is rather Alt/progressive and Ryan Adams is kind of alt-country-folk-rock combination, and is not nearly as critical and cynical with his take on America. Can you be more specific about what it is you connect between the two?
Based on my own take of those two...hmmm:
Josh Ritter (just introduced to him - brilliant songwriter)
Laurie Anderson's latest, Homeland
Elliot Smith (kind of down-key)
Jim White (he'll cover the South for you)
Sufijan Stevens (working on those midwest states)
Mark Kozelek (also on Sun Kil Moon & Red House Painters)
More classic takes on "America" that are less like the artist you mention:
Americana? So you're looking for music like Ryan Adams? Why dont you load/run Pandora and create a radio station (Ryan Adams Radio). Listen to it everyday for a few weeks & note what it's genre algorithms return and take it from there.
FWIW - ryan adams blows.
If you think the genre Americana is what you are interested in (I consider ryan adams cry-baby-pop) listen to Whiskeytown.
Here's a list of my faves that I dont consider AMERICANA just exceptionally good Rock/Folk music that have been in heavy rotation since their first releases where issued:
Uncle Tupelo (arguably the start of it all [I know Gram Parsons/Flying Burrito Bros - but come on!])
Son Volt/Jay Farrar/Gob Iron
Great Lake Swimmers
The Vulgar Boatmen
Chad Rex and the Victorstands
Drag The River
Great to see recommendations for Richard Buckner and Jeffrey Foucault! Both brilliant songwriters and Foccault is great on guitar. I heartily agree!
A few more in that come to mind:
Will Oldham (Bonnie "Prince" Billy / Palace Brothers / Palace Music)
If you really like Ryan Adams' "Heartbreaker," I'd suggest you pick up "Demolition," which is nothing at all like his first solo effort, but a damn good record, "Love is Hell," a fabulous Ryan Adams disc that the label didn't know what to do with, and two records he made with his band The Cardinals: "Cold Roses" and "Jacksonville City Nights."
If you should happen to fall for all of these -- and you well might, as the guy is a sometimes brilliant songwriter -- "Gold" is a glorious record from start to finish. It came right after "Heartbreaker," and was a total change in that it was often poppier than hell.
I had the great good fortune of seeing Adams a couple of times back in the day, and he's a truly rare talent.
Don't think anyone mentioned The Reivers yet. Back in the day, there was a lot of press to the effect that John Croslin was at the forefront of the "New Sincerity" movement of singer/songwriters you've nodded to here - and his bands' stuff added great harmonies, as well.
Sometimes things get a little too sincere, so I go for a shot of Webb Wilder.
PS Not sure Duane's choices are specifically "with the program", but they're very good ideas (especially Gatton), anyway.