Good morning, RL~
Funny you should ask....
I spoke this morning with yet another lumber yard - trying to find air-dried 5/4 Ash. My finished thickness is 1-1/8" - so even rough-sawn 4/4 is too thin.
The Oar pictured was made from kiln-dried stock - which a yard about an hour south of here carries. In fact, Zane Every put me onto Ghent Wood Products when I saw him at Tuckerton last September.
My first batch - in 2018 - was made from White Ash felled, sawn and planed by a succession of local friends.
I suspect kiln-dried is OK - but air-dried should be stronger viz. bending as the KD process can fracture the grain structure. I saw evidence of the in my last 2 Oars. One I filled - just a tiny void but nevertheless the last act of a desperate man - and the other I truncated by 4 inches.
My ideal would be 5/4 quarter sawn. One might think Ash would be readily available because of the recent invasion by the Emerald Ash Borer - but I'm having trouble finding a source. We have Ash here on the farm, but most is in hedgerows and so little chance of clear boards.
Maybe I need to corner the market on "traditional carbon fiber shovin' oars".....or maybe try some White Oak or even Sugar Maple (comparable viz. bending strength, stiffness and density).
All the best,
SJS