Mythology and Folklore of the Oak associates oaks with druidic and Gaelic pre-Roman culture, but does not investigate the revival of interest in that culture following the end of the Middle Ages. This Google cache describes a medieval painting in Cambridge which employs oak leaves and acorns as a decorative element.
This paper discusses the iconography of a section of Raphael’s monumental painting The School of Athens, originally executed for the residences of the Pope. The link includes a citation that in the painting, the figure of Epicurus is wearing a garland of oak, although a different citation also quoted describes the garland as ivy.
Finally, this link includes a direct quote from Pliny the Elder’s Natural History, “They select oak groves for the sake of that tree and will not perform any religious ceremony without its leaves. In fact the name ‘druid’ can even be derived from the word ‘oak’ if one employs a Greek etymology [drys, oak].”
Whatever the etymological relationship between the word druid and the fair dryads ensconced within the trees, it looks as though the symbolic etymology of the oak leaf as a badge of honor is factually pre-Roman, and continued to be used in Europe during the middle ages as well as the Renaissance.