Cheap oars are dead easy to make - if you have the right tools. I use the following:

• Tablesaw
• Belt sander
• Power Planer
• Chisel
• Clamps

You want to start with the shaft - it needs to be from fairly good wood - as few knots as possible. If there are knots, it is better if they are down at the blade end (the blades will provide support) or in the thicker sections.

Conventional wisdom says the length of the oar should be a bit less than 2x the beam, so the dimension of tapered section of the shaft is flexible. Also, you should keep the untapered section long enough to reach the oarlocks (where the force will be greatest.)

The oars don't have to be round, but for me, it is just as easy to make 'em round as it is to leave 'em square.