As Oracle adapter is not part of ActiveRecord in Rails 2.0 it is loaded after ActionPack class definitions. As a result at first CGI::Session::ActiveRecordStore::Session class is defined which inherits from ActiveRecord::Base and only afterwards OracleAdapter is loaded which adds write_lobs callback to ActiveRecord::Base but at this point it is not adding this callback to already defined Session class.
As a result when database session store is selected no session data are being stored in sessions table.
Attached patch adds missing after_save callback to Session class after OracleAdapter is loaded.