The major air base
Building the Navy’s Bases in World War II says a major air base was designated an OAK, and a smaller air base was known as an ACORN.
A directive of 4 July 1942, quoted in The Logistics of Advance Bases, used the name OAK No. 1 through OAK No. 7 for a group of four airfield units. The name ACORN No. 1 through ACORN No. 21 was for the parts of those groups. In each OAK, the lowest numbered ACORN was the carrier-deck unit.
The same history says that by 18 August 1942 the ACORN, rather than the OAK, was to be the basic unit, so the assembly could move. The OAK remained the name of the larger air-base group.