The BIR requires all taxpayers to issue OR's for all payments accepted / income generated. It doesn't matter if that client is not from the PH, you still have to issue an OR.
Now whether they want it or they want to keep it, that's up to them. :)
One important point: whatever income you report to the BIR should have a corresponding OR. That's one of the basic things they check should you be lucky enough to be chosen for an audit.