If you run large models and data sets, you might encounter memory problems. The JAVA environment of many systems requests less memory than possible. You can try to increase the heap memory of Onyx be starting Onyx from the terminal:
java -jar -Xmx2048m XXX.jar
with XXX being the file name of Onyx that you are running. The above command starts Onyx with 2GB of heap space. Depending on your system this number can be increased or must be decreased.