Stretch the hell out of a 2" rubber donut or go for a leather oar collar like the whitewater dories use.
Oh, and Sawyer's oars are 2.3" at the rope wraps. I am pretty sure they use the standard 2" donuts. You might call them and see if they'll sell you one of theirs if they are larger.
I'm sure they also have to stretch the hell out of their donuts, especially on the square tops.
Lube the shaft and donut with dish soap, or maybe try a big plywood board to ram it on. This is a great photo of the late Frank Harrington of Frank's Custom Oars in Oregon using a plywood bumper and some sort of collar to get the donut over the shaft/handle bump. (The cigarette might be helpful; I don't know as I don't smoke!)
Buying Locally: Frank’s Custom Oars | The Caddis Fly: Oregon Fly Fishing Blog