How to -create- performance issues? - oracle
This is a pretty broad topic that would largely depend on your specific needs.  

Here is a link to a fairly recent article about load testing tools.   It provides pros and cons of several different options:

http://www.pythian.com/blog/oracle-database-load-testing-tools-overview/

Personally, I like the Silly Little Oracle Benchmark (SLOB).  http://kevinclosson.wordpress.com/2013/05/02/slob-2-a-significant-update-links-are-here/