I recently had to deploy Hudson (the open-source continuous integration server) and CDR (a Grails application I built using Grails 1.0.1) at a client site onto OC4J 10g R3. As many of you old timey Grails developers may know Oracle is documented as having class loading issues with Grails applications. These issues are captured in an Oracle article written back in 2006. The fix, as documented by this article, is to set the deployment plan to "Search local classes first".
Ironically, my Grails application would not load with these settings. So I tried loading it without changing the deployment plan and Viola! It loaded with no issues.
The article did go to some good use, however. Hudson would not load without changing the setting to "Search local classes first."
Be warned, the current configuration UI is not identical to the one in the article.