I wouldn't use a rope with a separate sheath and core, it will keep the epoxy from oozing into the braid of the cord itself. It's tempting to use the cord with the kern and mantle, because it comes in cool patterns, but what you want looks more like traditional rope - just made of plastic.
The cord that I recommend is from my comments in the thread linked below. When you're buying the cord, do it in person and make sure you're buying the tightest braid possible in the diameter you've chosen. Also pull it TIGHT while you're wrapping it. Coat the shaft with epoxy (I'd use the new Gorilla Epoxy and make sure you have plenty of extra - it will take 1-2 standard $5 tubes per oar), wear gloves, and don't stop until you've tightly wrapped the entire section. Then have a cool sip of your beer.
The most important thing I can't stress enough: set up your boat how you will sit in it and then do so. Put the oars in your hands and space them like you like them. Mark the spot where your rubber stopper will go. Test wrap an oar without epoxy to see how long the wrapped section will be with the length of cord you are using (I liked 50 foot sections per oar, but some old school guys use a much smaller section) and mark the oars so that the rubber donut sits about 1/4-1/3 of the way into the wrap from the handle side.
It's a fun project, read those other posts and have a good time.
Rope Wrap: How ToŚ