Not easily. SAML is also making your life more difficult--when you use SAML you have to worry about three systems: the RP, Shib IDP and oxAuth. It can make for a toxic combination.
First of all, the RP gets the SAML assertion during authentication, and after that, does not hit the SAML IDP again. If you want more control over logout, you'll need to use a web proxy front end that can do that, perhaps the Shibboleth SP Apache filter. You can read more about Shibboleth logout issues here: https://wiki.shibboleth.net/confluence/display/IDP4/LogoutConfiguration
I'm closing this issue because logout is beyond the scope of community support. But if you find a good solution, please post it here for the benefits of others.