I am astonished that you could successfully move an oak tree that had a six inch diameter. Oak tree tap roots grow straight down and so moving an oak tree is fatal unless the entire tap root has been dug up. Normally this is impossible unless the tree is less than one year old. 
Was the tree planted correctly? The roots trimmed if they had been growing in a circle, inside a pot? The planting depth allowed the root flare at the base of the tree to be at soil level? See this web site.
https://extension.umn.edu/how/planting-and-transplanting-trees-and-shrubs
The cause of the brown leaves cannot be determined based on the pictures. If the tree has been planted correctly, it may still be stressed and infected by the Two-line chestnut borer beetle. See this web page. 
https://extension.umn.edu/tree-and-shrub-insects/metallic-wood-boring-beetles
Consult an arborist and/or carefully hand dig around the base of the tree to see if the roots are growing in a circle. 
https://www.treesaregood.org/findanarborist