Deep root irrigation will help if lack of water is the issue. You can hire an arbor master or do it yourself w/ a 2' piece of rebar 1/4" diameter. Or, after removing the soil from the roots, make a moat around the tree (from the dripline), place a hose inside the moat on a slow drip overnight. Search deep root irrigation to do it properly if you decide to DIY!
As the other reply said, once roots appear on top of ground (which happens when the tree does not get enough water...roots come up looking for a water source) they should not be covered with soil, especially young trees. When you see older oaks (50+ years) w/ mulch and bedding plants it's b/c they are established and most likely rec'd adequate water during the critical years of growth.