Splunk seems like an all around tool.
What is the advantage of incorporating the Ossec system into or with Splunk?
The reporting and searching is much easier using SPLUNK to look at & do searches on the OSSEC data. The newest version of SPLUNK and the OSSEC plugin give you a whole new set of features.
I've not updated to the 2.5.1 version, I'm still on 2.4, but I think I'll give it a try, x.x.1 just came out.
Users incorporate OSSEC alerts into Splunk to eliminate the need for a dedicated OSSEC web interface and allow for simplified incident analysis through aggregation and correlation.
Check out the app on Splunkbase: http://www.splunkbase.com/apps/All/4.x/app:Splunk+for+OSSEC+-+Splunk+v4+version
And an older blog detailing the value one company finds: http://www.ossec.net/main/splunk-ossec-integration
One that i can think of is that you can summarize data, or customize reports from Splunk, using OSSEC as an input.