oak applen. A round spongy gall on an oak leaf or twig, caused by the larvae of certain gall wasps.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 apple or oak galln (Botany) any of various brownish round galls on oak trees, containing the larva of certain waspsCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014oak? ap`ple n. any of various rounded galls produced on oaks. Also called oak? gall`. [1400?50] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.