Adding 'OR' in suitescript saved search

How do you do  an or... I have an existing saved search that has a condition.

I want to add an OR and another condition.

   var mySearch = search.load( { id : 'customsearch_cs_customer_invoice_overdue',

        columns : [ 'internalid',

                            'name',

                            'balance' ,

                            'transaction.partner'] });

        filters = mySearch.filters;

Doing this does gives an invalid filter error.

        filters.push(search.createFilter('OR' ));

This works on its own.....

        filters.push(search.createFilter({ name : 'partner',

                                            join : 'transaction',

                                            operator : search.Operator.ISEMPTY    }));