How to plant container oak trees: 

Dig hole 2x wider than dirt root ball/container.

Remove container. For plastic pots, completely remove.
For wooden boxes, place entire container into hole and simply remove sides.
The bottom wood panel can be buried beneath the tree ball and will decompose.

If roots appear tightly wound, loosen outer soil of rootball and spread/pull free some of the exterior roots so they can expand in outward directions in their new environment.

Center root ball in hole so top levels with the earth.

Backfill rest of hole with soil. Take care not to cover top “root flair” with soil, or tree might slowly suffocate. You want to be able to see where the base of the tree trunk slopes outwards toward the roots.

Make a “well of soil” in a ring above the covered hole. The circumference size of this ring should be roughly the same as, or larger than, the circumference of the root ball itself. Build it raised, so that when filled the ring can hold water like a small tub, and allow the water to slowly sink in above the root ball.

Watering newly-planted oak trees:

Water regularly (several times a week) until established, at least for 1st year. This is especially important during summer heat or drought conditions. The best way to water a tree is a slow deep soak rather than a fast, shallow watering. In other words, it is better to place a hose near its trunk and let it drip for hours, rather than turn the hose on full-blast for five minutes. Deep watering encourages deep root development, which enables the tree to grow larger. Once oak trees establish their deep roots, they are typically drought-resistant and no longer need this coddling.