To set up an Oil Service Reminder without having issued any certificates, first you'll need to add the customer and job address if you haven't done this already:
Create a New Customer, add their details and click "Save".
Click "Add Job Address".
Fill in the details and click "Save".
Click "Add Oil Appliance" to set a service date
No details for the appliance are needed at this stage
Set the "Service Period" to 6 or 12 months.
Then click in the "Service Due" box to select a date.