I did my 4.1m x 2.7m shed with 18mm OSB, like this http://www.wickes.co.uk/Wickes-General-Purpose-OSB3-Board-18-x-1220-x-2440mm/p/110517 (other suppliers are available) many years ago.
Very solid, and very walkable-on.
Then 2-layer felt; the first nailed, the second glued.