Please be sure and check the spelling for each. OakComplete is best used for all the oak tree stuff. OakFoliage is only for the leaf part if you want a different leaf texture. If not, just use OakComplete in all the texture slots.
For the moss I do believe you have to spell it 'Moss' with a capital M. Not spanish moss, just Moss. You may want to go through to the actual Material File labeled WombatFoliage and check the spelling if you need to.