oak leaf clustern. A decoration of bronze or silver oak leaves and acorns given to holders of various US military medals in recognition of acts entitling them to another award of the same medal.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.oak? leaf` clus?ter n. a bronze military decoration in the form of oak leaves, affixed to another decoration to signify a second award of the same medal. [1915?20, Amer.] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.