hi there,
i just googled "osepp uno" and the first link i took says that there is an important notice for rev1.1 & 1.2 osepp uno's
it says u have to select 1. Arduino Duemilanove Or 2. Nano w/Atmega 328, in the IDE and than it explains why etc..
take a look for your self: http://osepp.com/learning-centre/start-here/osepp-uno-arduino-compatible/
goodluck,
nick