The term 'oak fruit' refers to two different fruits that are produced by oak trees. The first is an ACORN, which is a nut with a hard shell and a single seed inside. Acorns are typically harvested in the fall and can be eaten raw or cooked. The second is a MAST, which is a type of nut that is larger than an ACORN and has a softer shell. Masts are usually harvested in the summer and can be used for food or animal feed.