Here you will find quick links where you can download the past papers of the USABO.
USABO Open Exam 2003 Answer Key
USABO Open Exam 2004 Answer Key
USABO Open Exam 2005 Answer Key
USABO Open Exam 2006 Answer Key
USABO Open Exam 2007 Answer Key
USABO Open Exam 2008 Answer Key
USABO Open Exam 2009 Answer Key
USABO Open Exam 2010 Answer Key
USABO Open Exam 2011 Answer Key
USABO Open Exam 2012 Answer Key
If you are interested in USABO 2014, USABO 2015, USABO 2016 past papers go to the Past papers section.
Source: https://www.usabo-trc.org.