I tried to run openproj under ubuntu 12.04 with the latest jdk7 from oracle. Openproj complains that the java vendor should be sun instead of oracle. So simply use the zip or tar.gz version of openproj from sourceforge, extract the files and change the following line in openproj.sh:
JAVA_OPTS="-Xms128m -Xmx768m"
to
JAVA_OPTS="-Djava.vendor=Sun -Xms128m -Xmx768m"
Then simply delete the .openproj folder in your homedir if you tried to start the program before. After that you should be able to run the program.