I have installed OATS, but when I try to record a test for Siebel through openscript, it is recorded in Standard Interactivity mode. Also in objects tests Siebel objects are recoginized as web objects.
I'm guessing that you try to create a functional test script, dont't you?
You need to activate the Siebel Test Automation library as indicated in the OpenScript User Guide (pdf in the C:\OracleATS\docs folder).
To do so, you need a licence (from licence.oracle.com) and you need to configure 2 parameters in Siebel.
You also need to start the Siebel application by adding a parameter to the URL SWECmd=AutoOn.
Again everything is explained in the standard doc.
Let us know if something goes wrong