To add a new field to your zendesk system to store Order Ref against a ticket follow these steps
1) In the bottom left of your zendesk agent page click "Admin" (cog icon)
2) Under MANAGE select "Ticket Fields"
3) in the top right click "Add Custom Field"
4) Select "Text" Field Type
5) Under "For Agents" enter a suitable title for example "OMS Order Ref"
6) if you want zendesk customers to see this make sure visible is ticked
7) Click Add Field to complete
8) Zendesk may take a little time to add this throughout the entire system