Need a few more details such as: What are you growing in, native soil or containers? How old is the tree and how high is the canopy above? As a general rule, it's a bad idea to grow under any tree, oak or otherwise since trees by nature produce a canopy that robs sunlight thereby reducing competition from other surrounding flora. The perceived additional security you get by growing in the understory can come at a high price - Diminished yields or even outright failure.