ALDEVAUX

The only leaf spot of oak that may look a bit like tar spot of maples that I know off is called Tubakia leaf spot. It usually occurs later in the season, mainly after the tree is stressed has it happened to yours. Here are some links that may help you identify the spots :

http://www.ppdl.org/dd/id/tubakia_leaf_spot-oak.html
http://www.uwex.edu/ces/wihort/gardenfacts/XHT1104.pdf
http://sickplants.tamu.edu/Factshee...kialeafspot.pdf
http://www.dnr.state.mn.us/fid/oct03/leafspot.html
http://www.uwex.edu/ces/wihort/gardenfacts/X1104.doc

Good luck