For some serious discussions of homebuilt oars, check out the Wooden Boat Forum:
The WoodenBoat Forum
It's a friendly site: you can post a question on building oars and get lots of informed advice.
Spruce and ash are the usual woods. Douglas fir is strong enough, perhaps. For one-piece solid wood oars, the crux is to pick really good stock (close, even vertical grain, clear with no knots or swirls). Biggest problem is the thin edges of the blade tend to split off, along the grain, which is why most oars are laminated and/or tipped with plastic guards.
If you make solid oars, think about coating them with a high-grade boatbuilders epoxy, first coat very thin so it soaks into the wood. You might reinforce the blades with f-glass cloth as well, with double or triple layers around the edges and tips (use f-glass tape).
If you go to the trouble, build at least 3 (one spare) or even 4 or 5.