How to apply oil when seasoning wood cutting board?
No need for elaborate application techniques (brushes, spray, etc.).

When I worked in an old style restaurant, the Chef had me:


clean the block with a wire brush**, then wipe free of dust
pour any cheap vegetable based oil directly on the block
use one of the towels from the linen service and vigorously rub the oil in
@wumpus idea for a next day second coat is good, we didn't do that


At home, you could use a painters wire brush, or if you are really motivated: wire brush wheel on an electric drill. Or a stainless steel pot scrubbing brush.
The one I used in the restaurant was similar to this:



The same brush was also used to clean the grill grate.

You can use any rag you want for the oil application. Strong paper towels would even work.


  I thought applying the oil with a towel would absorb too much of the oil making it wasteful.


For a home block, I'd guess you'd have 1 tbps absorbed by the block (that's a good thing). You'd probably loose 1 tbsp in the towel--the cost of doing it right.


  Also, after applying the oil to the cutting board and waiting a day or two, is it a good idea to clean it again with soap and water?


We just used very hot water on a towel to clean. Soap was forbidden. If it was soiled and permeated by something nasty, we would do the entire wire brush cycle again.



** Cleaning with the block with brush: I did it with a dry block. Some of the others would oil the block first, then liberally salt for the abrasive property, then wire brush it. My way gave similar results, but was easier.