Is a PhD from Open university worth the effort?
Do employers discriminate when hiring Open University PhD graduates ?

The only employers who would discriminate would be ones who know very little about research in whatever specialty you choose. Most PhDs at the open university are carried out in the same way as at other universities (you don't just get them for recording BBC2 through the night). Like most institutions the Open University is strong in some subject areas (eg Earth Sciences) and not so strong in others.