how to get rid of moss balls on oak trees?

Hire a professional who has the spray equipment big enough to reach the top of the trees. The fungicide is Kopercide (?sp) which is a copper based powder that is blue in color. You spray in the spring before the leaves form (so that you can spray the moss on the limbs). On live oaks, after the leaves drop in the spring and before the new leaves form. You get only 1 chance per year to time it right and it will have to be done for 2 consecutive years to get it all/most. You can spray some of the tree with a pump up sprayer and standing on a ladder, but the top 30 feet will still have the ball moss.  Be sure and get everything away from the spray and wash everything off with a spray.  The Kocide (Kopercide) is a little bit corrosive on metals.  Like metal roofs of homes. 

never heard of moss balls in oak trees ..but they do get balls of mistletoe