I can't think of such a motherboard. Your only options for ocing a mATX board would be what pH suggested. Even then you would not be able to achieve very high clock speeds as the power regulation system on such boards is barely adequate. Two or three stages of voltage regulation, weak caps.... just give up. It's not worth the effort. Instead try to be creative with slim desktop ATX cases. Silverstone makes a nice one here:
http://www2.newegg.com/Product/Produ...82E16811163006