To create Unit Of Measure(UOM) in openerp first install sale module and go to sale module menus in openerp and following these step given below
Step-1 go to Sales Configuration menu and click it.
Step-2 then go to Configuration Settings-> Product Section
Step-3 select products ->then click on Apply.
Step-4 Inventory Control Products->and click on Edit.
Step-5 Finally, Go to Unit of Measure->Purchase Unit of Measure ->click on Save